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Exposing and Using S-Log2 on the Sony A7s. Part One: Gamma and Exposure.

This document has been prepared independently of Sony. It is based on my own findings having used the camera and tested various exposure levels and methods. LUT’s to accompany this article can be found here.

If you find this useful please consider buying me a coffee or a beer. I’m not paid to write these articles.


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One of the really nice features of the Sony A7s is the ability to use different gamma curves and in particular the Sony S-Log2 gamma curve.

What are gamma curves?

All conventional cameras use gamma curves. The gamma curve is there to make the images captured easier to manage by making the file size smaller than it would be without a gamma curve. When TV was first developed the gamma curve in the camera made the signal small enough to be broadcast by a transmitter and then the gamma curve in the TV set (which is the inverse of the one in the camera) expanded the signal back to a normal viewing range. The current standard for broadcast TV is called “Recommendation BT-709”, often shortened to Rec-709. This gamma curve is based on standards developed over 60 years ago and camera technology has advanced a lot since then! Even so, almost every TV and monitor made today is made to the Rec-709 standard or something very similar. Many modern cameras can capture a brightness range, also known as dynamic range, that far exceed the Rec-709 standard.

The limitations of standard gammas.

As gamma effects the dark to light range of the image, it also effects the contrast of the image. Normal television gamma has a limited dynamic range (about 6 to 7 stops) and as a result also has a limited contrast range.

When shooting a high contrast scene with conventional gamma the brightest highlights and the darkest shadows cannot be recorded.
When shooting a high contrast scene with conventional gamma the brightest highlights and the darkest shadows cannot be recorded. The contrast on the TV or monitor will however be correct as the camera captures the same contrast range as the monitor is able to display.

Normally the gamma curve used in the camera is designed to match the gamma curve used by the TV or monitor. This way the contrast range of the camera and the contrast range of the display will be matched. So the contrast on the TV screen will match the contrast of the scene being filmed and the picture will look “normal”. However the limited dynamic range may mean that very bright or very dark objects cannot be accurately reproduced as these may exceed the gammas dynamic range.

Although the dynamic range of Rec-709 may not always capture the entire range of the scene being shot, as the gamma of the camera matches the gamma of the TV the contrast will appear correct.
Although the dynamic range of Rec-709 may not always capture the entire range of the scene being shot, as the gamma of the camera matches the gamma of the TV the contrast will appear correct.

The over exposure typical of a restricted range gamma such as Rec-709  is commonly seen as bright clouds in the sky becoming white over exposed blobs or bright areas on faces becoming areas of flat white. Objects in shade or shadow areas of the scene are simply too dark to be seen. But between the overexposed areas and any under exposure the contrast looks natural and true to life.

Typical limited Rec-709  exposure range. Contrast is good but the clouds are over exposed and look un-natural.
Typical limited Rec-709 exposure range. Contrast is good but the clouds are over exposed and look un-natural.

Log Gamma.

Log gamma, such as Sony’s S-Log2, allows the camera to capture a much greater brightness range or dynamic range than is possible when shooting with conventional television gamma. Dynamic range is the range from light to dark that the camera can capture or the range that the monitor or TV can display within one image. It is the range from the deepest blacks to the brightest whites that can be captured or shown at the same time.

There are some things that need to be considered before you get too excited about the possibility of capturing this much greater dynamic range. The primary one being that if the camera is set to S-log2 and the TV or monitor is a normal Rec-709 TV (as most are) then there is no way the TV can correctly display the image being captured, the TV just doesn’t have the range to show everything that the camera with it’s high range log gamma can capture accurately.

Fixed Recording Range For Both Standard and Log Gamma.

The signal range and signal levels used to record a video signal are normally described in percent. Where black is 0% and the brightest thing that can be recorded is normally recorded at 100% to 109%. Most modern video cameras actually record the brightest objects at 109%. The important thing to remember though is that the recording range is fixed. Even when you change gamma curve the camera is still constrained by the zero to 109% recording range. The recording range does not change whether you are recording Rec-709 or S-log2. So log gamma’s like S-Log2 must squeeze a much bigger signal range into the same recording range as used by conventional Rec-709 recordings.

Log gamma squeezes the scenes large range to fit in the camera's normal 0%-109% recording range.
Log gamma squeezes the scenes large range to fit in the camera’s normal 0%-109% recording range.

Recording S-Log2.

In order to record using S-log2 with the A7s you need to use a picture profile. The picture profiles give you several recording gamma options. For S-log2 you should use Picture Profile 7 which is already set up for S-log2 and S-Gamut by default (for information on gamuts see this article). In addition you should ALWAYS use the cameras native ISO which is 3200 ISO and it is normally preferable to use a preset white balance. Using any other ISO with S-log2 will not allow you to get the full benefit of the full 14 stops of dynamic range that S-log2 can deliver.

Grey Cards and White Cards.

Before I go further let me introduce you to grey and white cards in case you have not come across them before. Don’t panic you don’t have to own one, although I would recommend getting a grey card such as the Lastolite EzyBalance if you don’t have one. But it is useful to understand what they are.

The 90% White Card.

The 90% white card is a card or chart that reflects 90% of the light falling on it. This will be a card that looks very similar in brightness to a piece of ordinary white paper, it should be pure white, some printer papers are bleached or coloured very slightly blue to make them appear “brilliant white”  (as you will see later in many cases it is possible to use an ordinary piece of white paper in place of a 90% white card for exposure).

The Grey Card.

The 18% grey card, also often called “middle grey” card, is a card that reflects 18% of the light falling on it. Obviously it will appear much darker than the white card. Visually to us humans an 18% grey card appears to be half way between white and black, hence it’s other name, “middle grey”.

Middle grey is important because the average brightness level of most typical scenes tends to be around the middle grey brightness value. Another key thing about middle grey is that because it falls in the middle of our exposure range it makes it a very handy reference level when measuring exposure as it is less likely to be effected by highlight compression than a 90% white card.

Exposing White and Middle Grey.

Coming back to Rec-709 and conventional TV’s and monitors. If we want a piece of white paper to look bright and white on a TV we would record it and then show it at somewhere around 85% to 95% of the screens full brightness range. This doesn’t leave much room for things brighter than a white piece of paper! Things like clouds in the sky, a shiny car, a bright window or a direct light source such as a lamp or other light.  In order to make it possible for S-log2 to record a much greater dynamic range the recording level for white and mid tones is shifted down. Instead of recording white at 85%-95%, when using S-log2 it is recommended by Sony that white is recorded at just 59%. Middle grey moves down too, instead of being recorded at 41%-42% (the normal level for Rec-709) it’s recorded at just 32%. By recording everything lower this means that there is a lot of extra space above white to record all those bright highlights in any scene that would be impossible to record with conventional gammas.

To make room for the extra dynamic range and the ability to record very bright objects, white and mid tones are shifted down in level.
To make room for the extra dynamic range and the ability to record very bright objects, white and mid tones are shifted down in level by the S-log2 gamma curve. As a result, white, mid tones etc will be displayed darker than normally expected with conventional gamma.

As S-Log2 normally shifts a lot of the recording levels downwards, if we show a scene shot with S-Log2 that has been exposed correctly on a conventional Rec-709 TV or monitor it will look dark due to the lower recording levels. In addition it will look flat with very low contrast as we are now squeezing a much bigger dynamic range into the limited Rec-709 display range.

The on screen contrast appears reduced as the capture contrast is greater than the display contrast.
The on screen contrast appears reduced as the capture contrast is greater than the display contrast.

This on screen reduction in contrast and the darker levels are actually perfectly normal when shooting using log gamma, this is how it is supposed to look on a normal monitor or TV. So don’t be alarmed if when shooting using S-Log2 your images look a little darker and flatter than perhaps you are used to when shooting with a standard gamma. You will adjust the S-Log2 footage in post production to restore the brightness and contrast later.

Correctly exposed S-Log2 can look dark and washed out.
Correctly exposed S-Log2 can look dark and washed out.

The post production adjustment of S-Log2 is very important and one of the keys to getting the very best finished images. The S-Log2 recording acts as a digital negative and by “processing” this digital negative in post production (normally referred to as “grading”) we manipulate the large 14 stop dynamic range of the captured image to fit within the limited display range of a Rec-709 TV in a pleasing manner. This may mean pulling up the mid range a bit, pulling down the highlights and bit and generally shifting the brightness and colour levels of different parts of the image around  (see PART 2 for more post production information).

SLog-2 and 10 bit or 8 bit data.

Originally Slog-2 was designed for use on high end digital cinema cameras such as Sony’s F65 camera. These cameras have the ability to record using 10 bit data. A 10 bit recording can have up to around 1000 shades of grey from black to white. The A7s however uses 8 bit recording which only has a maximum of 235 shades from black to white. Normally 8 bit recording is perfectly OK as most transmission and display standards are also 8 bit. Shoot with an 8 bit camera and then display that image directly via an 8 bit system and nothing is lost. However when you start to grade and manipulate the image the difference between 8 bit and 10 bit becomes more significant. If you start to shift levels around, perhaps stretching out some parts of the image then the increased tonal resolution of a 10 bit recording helps maintain the very highest image quality. Photographers that have shot using both jpeg and raw will know how much more flexibility the 12 bit (or more) raw files have compared to the 8 bit jpeg’s. However they will also know that 8 bit jpeg’s can be also adjusted, provided you don’t need to make very large adjustments.

Contrary to popular belief heavy grading 8 bit footage does not necessarily lead to banding in footage across smooth surfaces except in extreme cases. Banding is more commonly a result of compression artefacts such as macro blocking. This is especially common with very highly compressed codecs such as AVCHD. The 50Mbps XAVC-S codec used in the A7s is a very good codec, far superior to AVCHD and as a result compression artefacts are significantly reduced, so banding will be less of an issue than with other lower quality codecs. If your going to shoot using S-Log2, some grading will be necessary and as we only have 8 bit recordings we must take care to expose our material in such a way as to minimise how far we will need to push and pull the material.

Getting Your Exposure Right.

When S-Log2 was developed the engineers at Sony produced tables that specified the correct exposure levels for s-Log2 which are:

exposure table1As you can see the nominal “correct” exposure for S-Log2 is a lot lower than the levels used for display on a typical Rec-709 TV or monitor. This is why correctly exposed s-log2 looks dark on a conventional TV. The implication of this is that when you grade your footage in post production you will have to shift the S-log2 levels up quite a long way. This may not be ideal with an 8 bit codec, so I decided to carefully test this to determine the optimum exposure level for the A7s.

Correct Exposure.

The panel of images below is from the A7s recording S-log2 and exposed at the Sony recommended “correct” 32% middle grey level. The correct exposure was determined using a grey card and an external waveform monitor connected to the cameras HDMI output. Then the S-log2 was corrected in post production to normal Rec-709 levels using a Look Up Table (LUT – more on LUT’s in part 2). You can also see the viewfinder display from the camera. If you click on the image below you can expand it to full size. Sorry about the shadow from the laundry line, I didn’t see this when I was shooting the test shots!

Correctly exposed S-Log2 from A7s.
Correctly exposed S-Log2 from A7s.

From this you can see just how dark and low contrast looking the original correctly exposed S-log2 is and how much more vibrant the corrected Rec-709 image is. I have also indicated where on the cameras histogram middle grey and white are. Note how much space there is to the right of white on the histogram. This is where the extra highlight or over exposure range of S-log2 can be recorded. When correctly exposed S-log2 has an exposure range of 6 stops above middle grey and 8 stops under.

Over Exposing or “Pushing” S-log2.

If we deliberately raise the exposure level above the Sony recommended levels (known as pushing the exposure), assuming you grade the image to the same final levels some interesting things happen.

For each stop we raise the exposure level you will have 1 stop (which is the same as 6db) less noise. So the final images will have half as much noise for each stop up you go. This is a result of exposing the image brighter and as a result not needing to raise the levels in post as far as you would if exposed at the normal level.

You will loose one stop of over exposure headroom, but gain one stop of under exposure headroom.

Bright highlights will be moved upwards into the most compressed part of the log gamma curve. This can result in a loss of texture in highlights.

Skin tones and mid tones move closer to normal Rec-709 levels, so less manipulation is need for this part of the image in post production.

This last point is important for the A7s with it’s 8 bit codec, so this is the area I looked at most closely. What happens to skin tones and textures when we raise the exposure?

Exposing at +1, +2 and +3 Stops.

Below are another 3 panels from the A7s, shot at +1 stop, +2 stops and +3 stops. Again you can click on the images if you wish to view them full size.

A7s S-Log2 over exposed by one stop.
A7s S-Log2 over exposed by one stop.
A7s S-Log2 over exposed by 2 stops.
A7s S-Log2 over exposed by 2 stops.
A7s S-Log2 over exposed by 3 stops.
A7s S-Log2 over exposed by 3 stops.

Looking at these results closely you can see that when you increase the exposure by 1 stop over the Sony specified correct level for S-log2 there is a very useful reduction in noise, not that the A7s is particularly noisy to start with, but you do get a noticeably cleaner image.

Below are 4 crops from the same images, after grading. I really recommend you view these images full size on a good quality monitor. Click on the image to view larger or full size.

Crops at different exposure of LUT corrected A7s S-log2 footage.
Crops at different exposure of LUT corrected A7s S-log2 footage.

The noise reduction at higher exposures compared to the base exposure is very clear to see if you look at the black edge of the colour checker chart (the coloured squares), although the difference between +2 and +3 stops is very small. You can also see further into the shadows in the +3 stop image compared to the base exposure. A more subtle but important effect is that as the exposure goes up the visible texture of the wooden clothes peg decreases. The grain can be clearly seen at the base level but by +3 stops it has vanished. This is caused by the highlights creeping into the more compressed part of the log gamma curve. The same thing is happening to the skin tones in the +3 stop image, there is some reduction of the most subtle textures.

From this we can see that for mid tones and skin tones you can afford to expose between 1 and 2 stops above the Sony recommended base level. More than 2 stops over and brighter skin tones and any other brighter textures start to be lost. The noise reduction gain by shooting between one and 2 stops over is certainly beneficial. The down side to this though is that we are reducing the over amount of exposure headroom.

As you raise the exposure level you reduce the over  exposure headroom.
As you raise the exposure level you reduce the over exposure headroom.

Given everything I have seen with this 8 bit camera my recommendation is to shoot between the Sony recommended base S-log2 level and up to two stops over this level. I would try to avoid shooting more than 2 stops over as this is where you will start to see some loss of texture in brighter skin tones and brighter textures.  Exactly where you set your exposure will depend on the highlights in the scene. If you are shooting a very bright scene you will possibly need to shoot at the Sony recommended level to get the very best over exposure headroom. If you are able to expose higher without compromising any highlights then you should aim to be up to 2 stops over base.

Determining The Correct Exposure.

The challenge of course is determining where your exposure actually is. Fortunately as we have seen, provided you in the right ball park, S-log2 is quite forgiving, so if you are a little bit over exposed it’s probably not going to hurt your images much. If you have a waveform monitor then you can use that to set your exposure according to the table below. If you don’t have proper white or grey cards you can use a piece of normal white paper. Although slightly less accurate this will get you very close to where you want to be. Do note that white paper tends to be a little brighter than a dedicated 90% reflectivity white card. If you don’t have any white paper then you can use skin tones, again a bit less accurate but you should end up in the right zone.

My suggested exposure levels for the Sony A7s. The "sweet spot" is from normal to +2 over.
My suggested exposure levels for the Sony A7s. The “sweet spot” is from normal to +2 over.

If you don’t have an external waveform monitor then you do still have some good options. Sadly although the camera does have zebras, these are not terribly useful for S-log2 as the lowest the zebras can go is 70%.

Light Meter: You could use a conventional photography light meter. If you do choose to use a light meter I would recommend checking the calibration of the light meter against the camera first.

Mark 1 Eyeball: You could simply eyeball the exposure looking at the viewfinder or rear screen but this is tricky when the image is very flat.

In Camera Metering: The cameras built in metering system, like the majority of DSLR’s is calibrated for middle grey. By default the camera uses multi-point metering to measure the average brightness of several points across the scene to determine the scenes average brightness and from there set the correct base S-log2 exposure.

Auto Exposure:

When you are using S-Log2, auto exposure in most cases will be very close to the correct base exposure if you use the default Multi-Zone exposure metering. The camera will take an average exposure reading for the scene and automatically adjust the exposure to the Sony recommended 32% middle grey exposure level based on this average. In the P, A and S modes you can then use the exposure compensation dial to offset the exposure should you wish. My recommendation would be to add +1 or +2 stops via the dial. Then observe the histogram to ensure that you don’t have any significant over exposure. If you do then reduce the exposure compensation. Lots of peaks to the far right of the histogram is an indication of over exposure.

Manual Exposure And Internal Metering.

If you are exposing manually you will see a small M.M. indication at the bottom of the LCD display with a +/- number. In the eyepiece viewfinder this appears as a scale that runs from -5 to +5, in S-log2 only the -2 to +2 part of the scale is used. In both cases this is how far the camera thinks you are away from the optimum exposure. + meaning the camera is over exposed, – meaning under.

A7s Viewfinder indications in manual exposure mode showing both M.M. offset from metered exposure and histogram.
A7s Viewfinder indications in manual exposure mode showing both M.M. offset from metered exposure and histogram.

In the image above we can see the M.M. indication is +0.3, in the eyepiece you would see a small arrow one bar to the right of “0” , indicating the cameras multi zone metering thinks the shot is just a little over exposed, even though the shot has been carefully exposed using a grey card and external waveform monitor. This error is probably due to the large amount of white in the shot, white shirt, white card, test charts with a lot of brighter than grey shades.  In practice an error of 0.3 of a stop is not going to cause any real issues, so even if this was exposed by setting  the exposure so that you have “M.M. 0.0” the exposure would be accurate enough. But it shows that multi point exposure averaging is easily confused.

The scene above is a fairly normal scene, not excessively bright, not particularly dark. If shooting a snow scene for example the cameras multi point averaging would almost certainly result in an under exposed shot as the camera attempts to bring the bright snow in the scene down to the average middle grey level. If shooting a well lit face against a very dark background then the averaging might try to bring the background up and the shot may end up overexposed.

If you want really accurate exposure then you should put the cameras metering system into the spot metering mode where instead of taking an average of various points across the scene the camera will just measure the exposure at the very center of the image.

A7s Spot Metering Mode.
A7s Spot Metering Mode.

You can then use a grey card to very accurately set the exposure. Simply place the circular shaped symbol at the center of the viewfinder display over a grey card and set the exposure so that M.M is 0.0 for the correct S-Log2 base exposure. To expose 1 stop over with a grey card, set M.M. +1.0 and two stops over M.M. +2.0 (not flashing, flashing indicates more than +2 stops).

Using Spot Metering to set exposure correctly for S-log2. MM 0.0.
Using Spot Metering to set exposure correctly for S-log2. MM 0.0.

One small issue with this is that the camera will only display a M.M. range of -2.0 to +2.0 stops. Provided you don’t want to go more than 2 stops over base then you will be fine with a grey card.

Using White Instead of Grey:

If you don’t have a grey card then you can use a 90% reflectivity white target. As white is 2 stops brighter than middle grey when S-Log2 is correctly exposed the 90% white should indicate M.M +2.0.

Using spot metering to set the correct exposure for S-Log2. M.M should read M.M +2.0 for a 90% reflectivity white target.
Using spot metering to set the correct exposure for S-Log2. M.M should read M.M +2.0 for a 90% reflectivity white target.

Once you have established the correct exposure you can then open the iris by 1 or two stops to increase the exposure. Or halve the shutter speed to gain a one stop brighter exposure. Each time you halve the shutter speed your exposure becomes one stop brighter, so divide the shutter speed by 4 to gain a 2 stop increase in exposure. As always you should observe the histogram to check for any over exposure. White peaks at the far right of the histogram or disappearing completely off the right of the histogram is an indication of over-exposure. In this case reduce your exposure back down towards the base exposure level (M.M 0.0 with a a grey card).

Exposure Summary:

I recommend using an exposure between the “correct” base S-Log2 exposure level of middle grey at 32% and two stops over this. I would not recommend going more than 2 stops over over base.

In the P, A and S auto exposure modes, when using the default multi-zone metering the camera will set the base S-log2 exposure based on the average scene brightness. For most typical scenes this average should be very close to middle grey. This exposure can then be increased (brightened) by up to 2 stops using the exposure compensation dial.

In manual exposure the “M.M.” number displayed at the bottom of the viewfinder display is how far you are from the correct base S-log2 exposure. M.M. +2.0 indicates +2 stops over base. If using multi zone metering (the cameras default) this exposure will be based on the scenes average brightness.

If you set the metering to “Spot” you can use a grey card centred in the image to determine the correct base exposure and up to 2 stops of over exposure via the M.M. indication when shooting manually.

In Part 2:

In part two I will take a look at grading the S-log2 from the A7s and how to get the very best from the S-log2 images by using Look Up Tables (LUT’s).

I welcome feedback on my articles. If you have any feedback please let me know. I will make this available as a PDF for download once part 2 is completed.

Major Update to my Cine-EI guide for the PMW-F55 and PMW-F5

I have just published a major update to my guide to Cine-EI on the PMW-F55 and F5. The guide now goes in to a lot more depth. I have tried to make it easy to understand but it is also quite technical, I have deliberately included the technical background stuff so that hopefully you will understand why Cine-EI and LUT’s work the way they do. I’ve added a whole new section on exposure methods for some of the different LUT’s as well as how to create your own LUT’s.

Please take a look if you use these cameras. Soon I will add a section on post production.

http://www.xdcam-user.com/2013/12/cine-ei-mode-when-recording-s-log23-and-raw-on-the-f5-and-f55/

LUT’s or LOOK’s when exposing via a LUT on the PMW-F5 or PMW-F55

First of all. You can use either, LUT’s or Looks. But there is a quite marked difference in the way they behave, especially if you use EI gain.

At the native ISO there is little to choose between them. But just to confirm my earlier suspicions about the way the 3D LOOK’s behave I ran a quick test.

I found that when you lower the EI gain, below native, the output level of the LOOK lowers, so that depending on the EI, the clipping, peak level and middle grey values are different. For example on my PMW-F5 at 500 EI the LC709TypeA LUT has a peak output (clipping) level of just 90% while at 2000 ISO it’s 98%. This also means that middle grey of the LOOK will shift down slightly as you lower the EI. This means that for consistent exposure at different low EI’s you may need to offset your exposure very slightly. It also means that at Native EI if the waveform shows peak levels at 90% you are not overexposed or clipped, but at low EI’s 90% will mean clipped Slog, so beware of this peak level offset.

When you raise the EI of the LOOKS, the input clipping point of the Look profile changes. For each stop of EI you add the LOOK will clip one stop earlier than the underlying Slog. For example set the LC709TypeA LUT to 8000 ISO (on my PMW-F5)  and the LOOK itself hard clips 2 stops before the actual SLog3 clips. So your LOOK will make it appear that your Slog is clipped up to 2 stops before it actually is and the dynamic range and contrast range of the LOOK varies depending on the EI, so again beware.

So, the Looks may give the impression that the Slog is clipped if you use a high ISO and will give the impression that you are not using your full available range at a low ISO. I suspect this is a limitation of 3D LUT tables which only work over a fixed 0 to1 input and output range.

What about the 1D LUT’s? Well the LUT’s don’t cover the full range of the Slog curves so you will never see all of your dynamic range at once. However I feel their behaviour at low and high EI’s is a little bit more intuitive than the level shifts and early clipping of the LOOKs.

The 1D LUT’s will always go to 109%. So there are no middle grey shifts for the LUT, no need to compensate at any ISO. In addition if you see any clipping below 109% then it means your SLog is clipping, for example if you set the camera to 500 ISO (on an F5), when you see the 709(800) LUT clipping at 105% it’s because the Slog is also clipping.

At High ISO’s you won’t see the top end of the SLog’s exposure range anyway because the LUT’s range is less than Slog’s range, but the LUT itself does not clip, instead highlights just go up above 109% and this is in my opinion more intuitive behaviour than the clipped LOOK’s that don’t ever quite reach 100% and clip at lower than 100% even when the Slog itself isn’t clipped.

At the end of the day use the ones that work best for you, just be aware of the limitations of both and that the LUT’s and LOOKs behave very differently. I suggest you test and try both before making any firm decisions.

Personally I prefer to use the 709(800) LUT for exposure as the restricted range matches that of most consumer TV’s etc so I feel this gives me a better idea of how the image may end up looking on a consumers TV. Also I find my Slog exposure more accurate as the LUT’s restricted range means you are more likely to expose within finer limits. In addition as noted above I fell the LUT’s behaviour is more predictable and intuitive at high and low EI’s than the LOOK’s.

In addition the higher contrast makes focus easier. I will often switch in and out of the LUT to look at how the Log is coping with any over exposure. This is my personal preference, but I do also use other LUT’s and Looks in particular the 709TypeA from time to time.

Exposing via LUT’s with the PMW-F5 and PMW-F55.

There is an ongoing and much heated debate on another forum about the practicalities of using the LUT’s or Looks built in to the PMW-F5 and PMW-F55 for setting the correct exposure of your SLog or Raw footage. In response to this I put together a very rough video demonstrating how this actually works.

Before watching the video, do please understand the following notes:

Correct exposure is normally determined by the level at which middle grey is recorded. This is true of both video and film production. Light meters are calibrated using middle grey. Expose with a light meter and you will find middle grey at the levels indicated below.

Different gamma curves may use different middle grey levels depending on the contrast required and the dynamic range of the gamma curve. Generally speaking, the greater the dynamic range, the lower middle grey must be set in order to leave room above middle grey for the extra dynamic range. This means that the relationship between middle grey and white will be different from curve to curve. Don’t always expect white to be some fixed value above middle grey. Some of the Sony looks for example LC709TypeA are very low contrast and while middle grey still sits at around 42% (The ITU standard for Rec-709 is 41.7%), because it is a low contrast, high dynamic range curve white is at a lower level, around 70%. The Hypergamma LUT grey points are given by the “G40″ or G33” number – G40 meaning middle grey at 40%.

When you take Slog or raw in to post production it is expected that the middle grey of the recordings will be at the correct nominal level (see chart below). If it is not, when you apply a post production Slog or raw LUT then the footage may appear incorrectly exposed. If you try to bring Slog or raw into an ACES workflow then ACES expects middle grey to be at the correct values. So it is important that your Slog or raw is exposed correctly if you want it to work as expected in post.

Correct exposure levels for Sony's Slog.
Correct exposure levels for Sony’s Slog.

Having said all of the above… If you are using CineEI and lowering or raising the EI gain from the native ISO then your Slog or raw will be exposed brighter or darker than the levels above. But I must assume that this is what you want as you are probably looking to adjust the levels in post to reduce noise or cope with an over exposure issue. You may need to use a correction LUT to bring your Slog levels back to the nominal correct levels prior to adding a post production LUT.

Anyway, here’s the video.

Cine-EI Mode when recording S-Log2/3 and raw on the F5 and F55.

INTRODUCTION:

This guide to Cine-EI is based on my own experience with the Sony PMW-F5 and F55. There are other methods of using LUT’s and CineEI. The method I describe below, to the best of my knowledge, follows standard industry practice for working with a camera that uses EI gain and LUT’s.

If you find the guide useful, please consider buying me a beer or a coffee. It took quite a while to prepare this guide and writing can be thirsty work.


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If you want you can download this guide as a PDF by clicking on this link: Ultimate Guide to CineEI on the PMW. I’d really appreciate a drink if your going to take away the PDF.

In this guide I hope to help you get the very best from the Cine EI mode on Sony’s PMW-F5 and PMW-F55 cameras. The cameras have two very distinct shooting mode, Cine EI and Custom Mode. In custom mode the camera behaves much like any other traditional video camera where what you see in the viewfinder is what’s recorded on the cards. In custom mode you can change many of the cameras settings such as gamma, matrix, sharpness to create the look you are after in-camera. “Baking-in” the look of your image in camera is great for content that will go direct to air or for fast turn around productions. But a baked-in look can be difficult to alter in post production. In addition it is very hard to squeeze every last drop of picture information in to the recordings in this mode.

The other mode, Cine-EI, is primarily designed to allow you to record as much information about the scene as possible. The footage from the camera becoming in effect a “digital negative” that can then be developed in post and the final highly polished look of the film or video created. In addition the Cine-EI mode mimics the way a film camera works giving the cinematographer the ability to rate the camera at different ISO’s to those specified by Sony. This can be used to alter the noise levels in the footage or to help deal with difficult lighting situations.

One further “non-standard” way to use Cine-EI is to use a LUT (Look Up Table) to create an in-camera look that can be baked in to the footage while you shoot. This offers an alternative to custom mode. Some users will find it easier to create a specific look for the camera using a LUT than they would by adjusting camera settings such as gamma and matrix.

MLUT’s, LUT’s and LOOK’s (all types of Look Up Tables) are only available in the Cine-EI mode.

THE SIMPLIFIED VERSION:

Before I go through all the “why’” and “hows” first of all let me just say that actually CineEI is easy. I’ve gone in to a lot of extra detail here so that you can full master the mode and the concepts behind it.

But in it’s simplest form, all you need to do is to turn on the MLUT’s. Choose the MLUT that you like the look of, or is closest to the final look you are after. Expose so that the picture in the viewfinder or on your monitor looks how you want and away you go.

Then in post production bring in your Slog footage. Apply the same LUT as you used when you shot and the footage will look as shot, only as the underlying footage is either raw or Slog you have a huge range of adjustment available to you in post.

THAT’S IT! If you want, it’s that simple.

If you want to get fancy you can create your own LUT and that’s really easy too (see the end of the document). If you want less noise in your pictures use a lower EI. I shoot using 800EI on my F5 and 640EI on the F55 almost all the time.

Got an issue with a very bright scene and strong highlights, shoot with a high EI.

Again, it’s really simple.

But anyway, lets learn all about it and why it works the way it works.

LATITUDE AND SENSITIVITY.

The latitude and sensitivity of the F5/F55, like most cameras is primarily governed by the latitude and sensitivity of the sensor. The latitude of the sensor in these cameras is around 14 stops. Adding different amounts of conventional camera gain or using  different ISO’s does not alter the sensors actual sensitivity to light, only how much the signal from the sensor is amplified. Like turning up or down the volume on a radio, the sound level gets higher or lower, but the strength of the radio signal is just the same. Turn it up loud and not only does the music get louder but also any hiss or noise, the ratio of signal to noise does not change, they BOTH get louder. Turn it up too loud and it will distort. If you don’t turn it up loud enough, you can’t hear it, but the radio signal itself does not change. It’s the same with the cameras sensor. It always has the same sensitivity, but with a conventional camera we can add or take away gain (volume control?) to make the pictures brighter or darker (louder?).

NATIVE ISO:

Sony’s native ISO ratings for the cameras, 1250ISO for the F55 and 2000ISO for the F5 are values chosen by Sony that give a good trade off between sensitivity, noise and over/under exposure latitude. In general these native ISO’s will give excellent results. But there may be situations where you want or need different performance. For example you might prefer to trade off a little bit of over exposure headroom for a better signal to noise ratio, giving a cleaner picture. Or you might need a very large amount of over exposure headroom to deal with a a scene with lots of bright highlights.

The Cine EI mode allows you to change the effective ISO rating of the camera.

With film stocks the manufacturer will determine the sensitivity of the film and give it an Exposure Index which is normally the equivalent of the films measured ASA/ISO.  It is possible for a skilled cinematographer to rate the film stock with a higher or lower ISO than the manufacturers rating to vary the look or compensate for filters and other factors. You then adjust the film developing and processing to give a correctly exposed looking image. This is a common tool used by cinematographers to modify the look of the film, but the film stock itself does not actually change it’s base sensitivity, it’s still the same film stock with the same base ASA/ISO.

Sony’s Cine EI and EI modes on Red and Alexa are very similar. While it has many similarities to adding conventional video camera gain, the outcome and effect can be quite different. If you have not used it before it can be a little confusing, but once you understand the way it works it is very useful and a great way to shoot. Again, remember that the actual sensitivity of the sensor itself never changes.

CONVENTIONAL VIDEO CAMERA GAIN.

Increasing conventional camera gain will reduce the cameras dynamic range as something that is recorded at maximum brightness (109%) at the native ISO would be pushed up above the peak recording level and into clipping if the conventional camera gain was increased because we can’t record a signal larger than 109%. But as the true sensitivity of the sensor does not change, the darkest object the camera can actually detect remains the same. Dark objects may appear a bit brighter, but there is still a finite limit to how dark an object the camera can actually see and this is governed by the sensors noise floor and signal to noise ratio (from the sensors own background noise). Any very dark picture information will be hidden in the sensors noise. Adding gain will bring up both the noise and darkest picture information, so anything hidden in the noise at the native ISO (or 0db) will still be hidden in the noise at a higher gain or ISO as both the noise and small signal are amplified by the same amount.

Using negative conventional gain or going lower than the native ISO may also reduce dynamic range as picture information very close to black may be shifted down below black when you subtract gain or lower the ISO. At the same time there is a limit to how much light the sensor can deal with before the sensor itself overloads. So even though reducing the ISO or gain may make the picture darker, the sensor clipping/overload point remains the same, so there is no change to the upper dynamic range, just a reduction in recording level.

See also this article on gain and dynamic range.

As Sony’s Slog2 and Slog3 are tailored to capture the cameras full 14 stop range this means that when shooting with Slog2 or Slog3 the gamma curve will only work as designed and deliver the maximum dynamic range when the camera is at it’s native ISO. At any other recording ISO or gain level the dynamic range will be reduced. IE: If you were to use SLog2 or SLog3 with the camera in custom mode and not use the native ISO by adding gain or changing the ISO, you will not get the full 14 stop range that the camera is capable of delivering.

EXPOSURE LEVELS FOR DIFFERENT GAMMA CURVES AND CONTRAST RANGES.

It’s important to know (or better still understand) that different gamma curves with different contrast ranges will require different exposure levels. The TV system that we use today is currently based around a standard known as Rec-709. This standard specifies the contrast range that a TV set or monitor can show and which recording levels represent which display brightness levels. Most tradition TV cameras are also based on this standard. Rec-709 does have some serious restrictions, the brightness and contrast range is very limited as these standards are based around TV standards and technologies developed 50 years ago. To get around this issue most TV cameras use methods such as a “knee” to compress together some of the brighter part of the scene into a very small recording range.

A traditional TV camera with a limited dynamic range compresses only a small highlight range.
A traditional TV camera with a limited dynamic range compresses only a small highlight range.

 

As you can see in the illustration above only a very small part of the recording “bucket” is used to hold a moderately large compressed highlight range. In addition a typical TV camera can’t capture all of the range in many scenes anyway. The most important parts of the scene from black to white is captured more or less “as is”. This leaves just a tiny bit of space above white to squeeze in a few highly compressed highlights. The signal from the TV camera is then passed directly to the TV and as the shadows, mid range and skin tones etc are all at more or less the same level as captured the bulk of scene looks OK on the TV/Monitor. Highlights may look a little “electronic” due to the very large amount of compression used.

But what happens if we want to record more of the scenes range? As the size of the recording “bucket”, the codec etc, does not change, in order to capture a greater range and fit it in to the same space, we have to re-distribute how we record things.

Recording a greater dynamic range into the same sized bucket.
Recording a greater dynamic range into the same sized bucket.

Above you can see that instead of just compressing a small part of the highlights we are now capturing the full dynamic range of the scene. To do this we have altered the levels that everything is recorded at. Blacks and shadows are a little lower, greys and mids are a fair bit lower and white is a lot lower. By bringing all these levels down we make room for the highlights and the really bright stuff to be recorded without being excessively compressed.

The problem with this though is that when you output the picture to a monitor or TV it looks odd. It will lack contrast as the really bright stuff is displayed at the same brightness as the conventional 709 highlights. White is now only as bright as faces would be with a conventional TV camera and Faces are only a little bit above the middle grey level.

This is how Slog works. By re-distributing the recording levels we can squeeze a much bigger dynamic range into the same size recording bucket. But it won’t look quite right when viewed directly on a standard TV or monitor. It may look dark and certainly a bit washed out.

I hope you can also see from this that whenever the cameras gamma curve does not match that of the TV/Monitor the picture might not look quite right. Even when correctly exposed white may be at different levels, depending on the gamma being used, especially if the gamma curve has a greater range than the normal Rec-709 used in old school TV cameras.

THE CORRECT EXPOSURE LEVELS FOR SLOG-2 and SLOG-3.

Lets look at the correct exposure levels for SLog-2 and SLog-3. As these gamma curves have a very large dynamic range the recording levels that they use are different to the levels used by the normal 709 gamma curve used for conventional TV. As a result when correctly exposed, Slog looks dark and low contrast on a conventional monitor or in the viewfinder. The table below has the correct levels for middle grey and 90% reflectance white for the different types of Slog.

Correct exposure levels for Sony's Slog.
Correct exposure levels for Sony’s Slog.

The white level in particular is a lot lower than we would normally use for TV gamma. This is done to make extra space above white to fit in the extended range that the camera is capable of capturing, all those bright highlights, bright sky and clouds and other things that cameras with a smaller dynamic range struggle to capture.

Let’s now take a look at how to set the correct starting point exposure for SLog-3. You can use a light meter if you wish, but if you do want to use a light meter I would first suggest you check the calibration of the light meter by using the grey card method below and comparing the what the light meter tells you with the results you get with a grey or white card.

The most accurate method is to use a good quality grey card and a waveform display. For the screen shots seen here I used a Lastolite EzyBalance Calibration Card. This is a pop up grey card/white card that fits in a pocket but expands to about 30cm/1ft across giving a decent sized target. It has middle grey on one side and 90% reflectance white on the other. With the MLUT’s off, set the exposure so that the grey card is exposed at the appropriate level (see table above).

Note: To get the waveform to display you must have BOTH the SDI SUB and Viewfinder MLUT’s OFF or BOTH ON. The waveform is turned on and off under the  “VF” – “Display ON/OFF” – “Video Signal Monitor” settings of the main menu. Sadly the cameras built in waveform display is not the best so it may help to use an external monitor with a better waveform display.

Setting the correct exposure for Slog-3 using a grey card. Middle grey should be 41%
Setting the correct exposure for Slog-3 using a grey card. Middle Grey should be 41%

If you don’t have access to a better waveform display you can use a 90% reflectance white card and zebras. By setting up the Zebras with a narrow aperture window of around 3% you can get a very accurate exposure assessment for various shades of white. For SLog-3 set the Zebras to 3% aperture and the level at 61%. Sadly the zebras don’t go below 60%. For Slog-2 using 60% will be accurate enough, a 1% error is not going to do any real harm. You can use exactly the same method for S-Log2 just by using the SLog-2 levels detailed in the chart above.

Setting up the Zebras to measure S-Log3 exposure of white card (90% reflectance white card).
Setting up the Zebras to measure S-Log3 exposure of white card (90% reflectance white card).
Correct exposure for S-Log3 when using a 90% reflectance white target.
Correct exposure for S-Log3 when using a 90% reflectance white target.

The image above shows the use of both the Zebras and Waveform to establish the correct exposure level for S-Log3 when using a 90% reflectance white card or similar target. Please note that very often a piece of white paper or a white car etc will be a little bit brighter than a calibrated 90% white card. If using typical bleached white printer paper I suggest you add around 4% to the white values in the above chart to prevent under exposure.

SO HOW DOES CINE-EI WORK?

Cine EI is selected in the Base Settings page. It works in YPbPr, RGB and Raw main operation modes.
Cine EI is selected in the Base Settings page. It works in YPbPr, RGB and Raw main operation modes.

Cine-EI (Exposure Index) works differently to conventional camera gain. It’s operation is similar in other cameras that use Cine-EI or EI gain such as the F5, F55, F3, F65, Red or Alexa. You enable Cine-EI mode in the camera menus Base Settings page. On the F5 and F55 it works in YPbPr, RGB and RAW modes.

IMPORTANT: In the Cine-EI mode the ISO of the recordings remains fixed at the cameras native ISO (unless baking in a LUT,  more on that later). By always recording at the cameras native ISO you will always have 14 stops of dynamic range.

YOU NEED TO USE A LUT:

Important: For Cine-EI mode to work as expected you should monitor your pictures in the viewfinder or via the SDI/HDMI output through one of the cameras built in MLUT’s (Look Up Table), LOOK’s or User3D LUT’s. So make sure you have the MLUT’s turned on. If you don’t have a LUT then it won’t work as expected because the EI gain is applied to the cameras LUT’s.  At this stage just set the MLUT’s to on for the Sub&HDMI output and the Viewfinder out.

For CineEI to work correctly you MUST turn on the LUT's for the Viewfinder and/or Sub&HDMI output.
For CineEI to work correctly you MUST turn on the LUT’s for the Viewfinder and/or Sub&HDMI output.
You can turn the MLUT's on and off by pressing the CAMERA button until you see the MLUT controls, then use the hotkeys to turn the MLUT's on and off.
You can turn the MLUT’s on and off by pressing the CAMERA button until you see the MLUT controls, then use the hotkeys to turn the MLUT’s on and off.

EXPOSING VIA THE LUT/LOOK.

At the cameras native ISO (2000 on F5, 1250 on F55), when shooting via a LUT you should adjust your exposure so that the picture in the viewfinder looks correctly exposed. If the LUT is correctly exposed then so too will the S-log. As a point of reference, middle grey for Rec-709 and the 709(800) LUT should be at, or close to 42%.

This is really quite simple, generally speaking when using a LUT, if it looks right, it probably is right. However it is worth noting that different LUT’s may have slightly different optimum exposure levels. For example the 709(800) LUT is designed to be a very close match to the 709 gamma curve used in the majority of monitors, so this particular LUT is really simple to use because if the picture looks normal on the monitor then your exposure will also be normal.

Correct exposure of Middle Grey for the 709(800) MLUT. Middle Grey should be 42%. 90% will be 90%.
Correct exposure of Middle Grey for the 709(800) MLUT. Middle Grey should be 42%. 90% will be 90%.
Correct exposure of the 709(800) LUT using a 90% white card, white will be 90%. You can use zebras at 90% to check this level.
Correct exposure of the 709(800) LUT using a 90% white card, white will be 90%. You can use zebras at 90% to check this level.

The above images show the correct exposure levels for the 709(800) LUT. Middle grey should be 42% and 90% white is… well 90%. Very simple and you can easily use zebras to check the white level by setting them to 90%. As middle grey is where it normally is on a TV or monitor and white is also where you would expect to see it, when using the 709(800) LUT, if the picture looks right in the viewfinder then it generally is right. This means that the 709(800) LUT is particularly well suited to being used to set exposure as a correctly exposed scene will look “normal” on a 709 TV or monitor.

Many of the other LUT’s and Looks however capture a contrast range that far exceeds the Rec-709 standard used in most monitors. So you may need to adjust your exposure levels slightly to allow for this.

The LC709-TypeA Look is very popular as a LUT for the PMW-F5 and F55 as it closely mimics the images you get from an Arri Alexa (“type A” = type Arri).

The “LC” part of the Look’s name means Low Contrast and this also means – big dynamic range. Whenever you take a big dynamic range (lots of shades) and show it on a display with a limited dynamic range (limited shades) all the shades in the image get squeezed together to fit into the monitors limited range and as a result the contrast gets reduced. This also means that middle grey and white are also squeezed closer together. With conventional 709 middle grey would be 42% and white around 80-90%, but with a high dynamic range/low Contrast gamma curve white gets squeezed closer to grey to make room for the extra dynamic range. This means that middle grey will remain close to 42% but white reduces to around 72%. So for the LC709 Looks in the F5/F55 optimum exposure is to have middle grey at 42% and white at 72%. Don’t worry too much if you don’t hit those exact numbers, a little bit either way does little harm.

Correct white level for the LC709 LOOK's. White should be around 72%
Correct white level for the LC709 LOOK’s. White should be around 72%

 

RECOMMENDED LUT EXPOSURE LEVELS.

Here are some white levels for some of the built in LUT’s. The G40 or G33 part of the HG LUT’s is the recommended value for middle grey. Use these levels for the zebras if you want to check the correct exposure of a 90% reflectance white card. I have also include an approximate zebra value for a piece of typical white printer paper.

709(800) = Middle Grey 42%. 90% Reflectance white 90%, white paper 92%.

HG8009(G40) = Middle Grey 40%. 90% Reflectance white 83%, white paper 86%.

HG8009(G33) = Middle Grey 33%. 90% Reflectance white 75%, white paper 80%.

The “LC709” LOOK’s = Middle Grey 42%. 90% Reflectance white 72%, white paper 77%.

DONT PANIC if you don’t get these precise levels! I’m giving them to you here so you have a good starting point. A little bit either way will not hurt. Again, generally speaking if it looks right in the viewfinder or on your monitor screen, it is probably close enough not to worry about it.

If you ever need to confirm the correct levels for any given LUT or Look it’s really easy. Put the camera in to CineEI.  Turn OFF the MLUT’s (remember you can turn LUT’s on and off from the cameras side information screen by pressing the CAMERA menu button until you see the LUT options at the bottom of the display). With the MLUT’s OFF use a grey card or white card (or maybe your calibrated light meter) to set the exposure for the Slog curve you have chosen.

Once you have established the correct exposure for the SLog, without adjusting anything else, turn on the MLUT’s, ensure the camera is at the native ISO and choose the LUT or LOOK that you want to check.  Now you can measure the grey and white point for the LUT/Look you have chosen and see on the monitor or in the viewfinder what the correct exposure looks like via the LUT. It probably won’t be vastly different from normal 709 in most cases, especially middle grey this tends to stay very close to 42%, but it’s useful to do this check if you are at all unsure.

If  you can, use a LUT, not a LOOK. That’s my recommendation, not a hard and fast rule but if you’re new to CineEI, LUT’s and Looks please read on as to why I say this. Otherwise skip on to Baking-in the LUT/LOOK.

I recommend that for exposure evaluation you should normally try to use one of the cameras built in  MLUT’s not the LOOK’s. Especially if you are new to LUT’s and Looks. This is because the LUT’s behave differently to the Looks when you use a high or low EI.

I found that when you lower the EI gain, below native, the output level of the LOOK lowers, so that depending on the EI, the clipping, peak level and middle grey values are different. For example on my PMW-F5 at 500 EI the LC709TypeA LUT has a peak output (clipping) level of just 90% while at 2000 ISO it’s 98%. This also means that middle grey of the LOOK will shift down slightly as you lower the EI. This means that for consistent exposure at different low EI’s you may need to offset your exposure very slightly (it is only very slight). It also means that at Native EI if the waveform shows peak levels at 90% you are not overexposed or clipped, but at low EI’s 90% will mean clipped Slog, so beware of this peak level offset with the LOOK’s.

When you raise the EI of the LOOKS, the input clipping point of the Look profile changes. For each stop of EI you add the LOOK will clip one stop earlier than the underlying Slog. For example set the LC709TypeA LUT to 8000 ISO (on my PMW-F5)  and the LOOK itself hard clips 2 stops before the actual SLog3 clips. So your LOOK will make it appear that your Slog is clipped up to 2 stops before it actually is and the dynamic range and contrast range of the LOOK varies depending on the EI, so again beware.

So, the Looks may give the impression that the Slog is clipped if you use a high ISO and will give the impression that you are not using your full available range at a low ISO. I suspect this is a limitation of 3D LUT tables which only work over a fixed 0 to1 input and output range.

What about the 1D LUT’s? Well the built in LUT’s don’t cover the full range of the Slog curves so you will never see all of your dynamic range at all at once. However I feel their behaviour at low and high EI’s is a little bit more intuitive than the level shifts and early clipping of the LOOKs.

The 1D LUT’s will always go to 109%. So there are no middle grey shifts for the LUT, no need to compensate at any ISO. In addition if you see any clipping below 109% then it means your SLog is clipping, for example if you set the camera to 500 ISO (on an F5), when you see the 709(800) LUT clipping at 105% it’s because the Slog is also clipping.

At High ISO’s you won’t see the top end of the SLog’s exposure range anyway because the 1D LUT’s range is less than Slog’s range, but the LUT itself does not clip, instead highlights just go up above 109% where you can’t see them and this in my opinion is more intuitive behaviour than the clipped LOOK’s that don’t ever quite reach 100% and clip at lower than 100% even when the Slog itself isn’t clipped.

At the end of the day use the ones that work best for you, just be aware of the limitations of both and that the LUT’s and LOOKs behave very differently. I suggest you test and try both before making any firm decisions, but my recommendation is to use the LUT’s rather than the LOOK’s when judging exposure.

Using the built in MLUT’s?

There are 5 built in MLUT’s: P1 709(800), P2 HG8009G40, P3 HG8009G33, P4 Slog2, P5 Slog3. You can only select the Slog2 LUT when the camera is set to SLog2 in the base settings, the same for the SLog3 LUT. You can also create your own 1D LUT’s in Sony’s Raw Viewer software and user 3D LOOK’s in most grading suites but that’s a whole other subject that I’m not going to cover right here (see this article for user 3D Look creation or go to the additional LUT creation section at the bottom of this document), for now lets just consider the built in LUT’s.

All 3 of the other built in LUTs have an 800% exposure range. The camera itself  has a 1300% exposure range when your shooting in CineEI, Raw,  SLog2 or SLog3 (1300% more than standard gamma). So if you want to see your full exposure range then you should select SLog as your LUT or turn the LUT’s off. However the pictures will be flat looking and lack contrast, which makes accurate focussing harder and you must set your exposure using the SLog levels given above, so in addition Slog 2 will look dark. Note that if you press the “camera” button by the side LCD screen 2 times you can use the hot keys around the LCD to change LUT and turn the LUT’s on and off.

If you use MLUT’s P1, P2 orP3 then the viewfinder pictures will have near “normal” contrast. Your exposure levels will be more normal looking (although P3 should have middle grey at 33% so should look a touch darker than normal) but you won’t be seeing the full recorded range, only 800% out of the possible 1300% is displayed, so some things might look clipped in the viewfinder while the actual recording is not. I suggest switching  the LUT’s off momentarily to check this, or connect a second monitor to the Main SDI to monitor the non LUT output. Do note that if using Slog2/Slog3 as a LUT and at a positive EI ISO you won’t see your full recording range. You will still see up to a stop more than the 800% LUT’s but at high EI’s the Slog2 or Slog3 LUT will clip slightly before the camera recordings. At low EI’s the Slog LUT’s will clip at the same time as the camera, but the level of the clipping point on any waveform display via the LUT output will be lower and this can be a little confusing. Unfortunately if you turn off the LUT’s you can’t get the cameras built in waveform display. This is where an external monitor with waveform becomes very handy to monitor the non LUT native ISO Slog output from the main HDSDI output.

Personally I prefer to use the 709(800) LUT for exposure as the restricted range matches that of most consumer TV’s etc so I feel this gives me a better idea of how the image may end up looking on a consumers TV. The slightly restricted range will help highlight any contrast issue that may cause problems in post. It’s often easier to solve these issues when shooting rather than leaving it to later. Also I find my Slog exposure more accurate as the LUT’s restricted range means you are more likely to expose within finer limits. In addition as noted above I fell the LUT’s behaviour is more predictable and intuitive at high and low EI’s than the LOOK’s.

BAKING IN THE LUT/LOOK.

When shooting using a high or low EI, the EI gain is added or subtracted from the LUT or LOOK, this makes the picture in the viewfinder or monitor fed via the LUT brighter or darker depending on the EI used. In Cine-EI mode you want the camera to always actually record the raw and S-Log2/S-log3 at the cameras native ISO (1250 ISO for F55 or 2000 ISO for F5). So normally you want to leave the LUT’s OFF for the internal recording. Just in case you missed that very important point: normally you want to leave the LUT’s OFF for the internal recording!

Normally in CineEI you need to ensure that the LUT is OFF for the Main&Internal so that there is no LUT added to the SxS recordings.
Normally in CineEI you need to ensure that the LUT is OFF for the Main&Internal so that there is no LUT added to the SxS recordings.

Just about the only exceptions to this might be when shooting raw or when you want to deliberately record with the LUT/Look baked in to your SxS recordings. By “baked-in” I mean with the gamma, contrast and color of the LUT/Look permanently recorded as part of the recording. You can’t remove this LUT/look later if it’s “baked-in”.

No matter what the LUT/Look settings, if you’re recording raw on the R5 raw recorder the raw is always recorded at the native ISO.  But the internal SxS recordings are different. It is possible, if you choose, to apply a LUT/LOOK to the SxS recordings by setting the “Main&Internal” MLUT to ON. The gain of the recorded LUT/LOOK will be altered according to the CineEI gain settings. This might be useful to provide an easy to work with proxy file for editing, with the LUT/LOOK baked-in while shooting raw. Or as a way create an in-camera look or style for material that won’t be graded. Using a baked-in LUT/LOOK for a production that won’t be graded or only have minimal grading is an interesting alternative to using Custom Mode that should be considered for fast turn-around productions.

In most cases however you will probably not have a LUT applied to your primary recordings. If shooting in S-Log2 or S-Log3 you must set MLUT – OFF for “Main&Internal” See the image above. With “Main&Internal MLUT OFF” the internal recordings, without LUT, will be SLog2 or Slog3 and at the cameras native ISO.

You can tell what it is that the camera is actually recording by looking in the viewfinder. At the center right side of the display there is an indication of what is being recorded on the cards. Normally for Cine-EI this should say either SLog2 or Slog3. If it indicates something else, then you are baking the LUT in to the internal recordings.

The internal recording gamma is shown on the right of the VF. This is recording SLog-3
The internal recording gamma is shown on the right of the VF. This is recording SLog-3

 

The indication here shows that the 709(800) LUT is being baked-in to the internal recordings.
The indication here shows that the 709(800) LUT is being baked-in to the internal recordings.

CHANGING THE EI.

Latitude Indication.

At the native ISO you have 6 stops of over exposure latitude. This is how much headroom your shot has. Your over exposure latitude is indicated on the cameras side LCD panel as highlight latitude.

High light range indicated in "High Latitude" box as 6E or +6 stops. EI is 2000 (native F5 ISO)
High light range indicated in “High Latitude” box as 6E or +6 stops. EI is 2000 (native F5 ISO)

REDUCING THE EI.

So what happens when you halve the EI gain to 1000EI?  1 stop of ISO will subtracted from the LUT. As a result the picture you see via the LUT becomes one stop darker (a good thing to know is that 1 stop of exposure is the same as 6db of gain or a doubling or halving of the ISO). So the picture in the viewfinder gets darker. But also remember that the camera will still be recording at the native ISO (unless baking-in the LUT).

With the EI reduced to 1000EI from the native 2000EI the High Latitude decreases to 5.0E or +5 stops.
With the EI reduced to 1000EI from the native 2000EI the High Latitude decreases to 5.0E or +5 stops.

As you can see from the side panel indication, the cameras highlight latitude decreases by 1 stop.

Why does this happen and whats happening to my pictures?

First of all lets take a look at the scene, as seen in the cameras viewfinder when we are at the native EI (This would be 1250 on the F55 and 2000ISO on an F5) and then with the EI changed one stop down so it becomes 500EI on F55 or 1000EI on the F5. The native ISO on the left, the one stop lower EI on the right.

2000EI and 1000EI as seen in the viewfinder with NO exposure change.
2000EI and 1000EI as seen in the viewfinder of an F5 with NO exposure change.

So, in the viewfinder, when we lower our EI by one stop (halving the EI) the picture becomes darker by 1 stop. Note that if you were using the waveform display or histogram the indicated levels would also become lower. The waveform, histogram and zebras all measure the output from the LUT or the image seen in the viewfinder. So as this becomes one stop darker, they would also read 1 stop darker/lower. If using an external monitor with a waveform display connected to the SDI SUB out (SDI’s 3&4) or HDMI and the LUT is enabled for “Sub&HDMI” this too would get darker and the levels decrease by one stop.

What do you do when you have a dark picture? Well most people would normally compensate for a dark looking image by opening the iris to compensate. As we have gone one stop darker with the EI gain, to return the viewfinder image back to the same brightness as it was at the native EI you would open the iris by one stop.

If using a light meter you would start with the meter set at 2000/1250 ISO and set your exposure according to the what the meter tells you. Then you reduce your EI gain on the camera (the viewfinder image gets darker). Now you also change your ISO on the light meter to the new EI ISO. The light meter will then tell you to open the iris by one stop.

So now, after reducing the EI by one stop and then compensating by opening the iris by 1 stop, the viewfinder image is the same brightness as it was when we started.

But what’s happening to my recordings?

Remember the recordings, whether on the SxS card (assuming the Main&Internal LUT is OFF) or RAW always happens at the cameras native ISO (2000 on the F5 and 1250 on the F55), no matter what the EI is set to. As a result, because we opened the iris by 1 stop to compensate for the dark viewfinder or new light meter reading the recording will have become 1 stop brighter. Look at the image below to see what we see in the viewfinder alongside what is actually being recorded. The EI offset exposure as seen in the viewfinder (left hand side) looks normal, while the actual native ISO recording (right hand side) is 1 stop brighter.

VF-and-Internal
At 1000EI the Viewfinder image on the left is 1 stop darker than the actual recorded image (on the right) which is recorded at the native 2000 ISO (F5).

How does this help us, what are the benefits?

When I take this brighter recorded image in to post production I will have to bring the levels back down to normal as part of the grading process. As I will be reducing my levels in post production by around 1 stop (6db) any noise in the picture will also be reduced by 6db. The end result is a picture with 6db less noise than if I had shot at the native ISO. Another benefit may be that as the scene was exposed brighter I will be able to see more shadow information.

Is there a down side to using a low EI?

Because the actual recorded exposure is brighter by one stop I have one stop less headroom. However the F5 and F55 have an abundance of headroom so the loss of one stop is often not going to cause a problem. I find that going between 1 and 1.5 EI stops down rarely results in highlight issues. But when shooting very high contrast scenes and using a low EI it is worth toggling the LUT on and off to check for clipping in the SLog image. This can be done from the side panel of the camera by pressing the “Camera” button until you see the MLUT controls on the side display and turning the MLUT on or off.

What is happening to my exposure range?

What you are doing is moving the mid point of your exposure range up in the case of a lower EI. This allows the camera to see deeper into the shadows but reduces the over exposure latitude. The reverse is also possible. If you use a higher EI you shift your mid point down. This gives you more headroom for dealing with very bright highlights, but you won’t see as far into the shadows and the final pictures will be a little noisier as in post production the overall levels will have to be brought up to compensate for the darker overall recordings.

Cine-EI allows us to shift our exposure mid point up and down.  Lowering the EI gain gives you a darker VF image so you tend to overexpose the actual recording which reduces over exposure headroom but increases under exposure range (and improves the signal to noise ratio). Adding EI gain gives a brighter Viewfinder image which makes you underexpose the recordings, which gives you more headroom but with less underexposure range (and a worse signal to noise ratio).

Cine EI moves the exposure mid point up and down.
Cine EI moves the exposure mid point up and down.

Post Production.

When shooting raw information about the EI gain is stored in the clips metadata. The idea is that this metadata can be used by the grading or editing software to adjust the clips exposure level in the edit or grading application so that it looks correctly exposed (or at least exposed as you saw it in the viewfinder via the LUT). The metadata information is recorded alongside the XAVC or SSTP footage when shooting SLog2/3. However, currently few edit applications or grading applications use this metadata to offset the exposure, so S-Log2/3 material may look dark/bright when imported into your edit application and you may need to add a correction to return the exposure to a “normal” level. As the footage is log you should use log corrections to get the very best results. As an alternative you can use a correction LUT to move the exposure up and down like the ones for SLog2 available here, created by cameraman and DP Ben Turley. http://www.turley.tv. Sony’s Raw Viewer software does correctly read the Slog2/3 metadata and will automatically add any required offsets. If shooting raw the majority of grading and editing applications will correctly read the metadata in the raw footage and apply the correct exposure offset, so raw normally looks correctly exposed.

See also this article for more information on EI gain and raw on the F5 and F55.

If shooting raw then you may choose to add the S-Log2/3 LUT to your internal SxS recordings. This will then add the EI-gain to the internal SxS recordings, so they will become brighter/darker as if applying actual gain while only the raw recordings remain at the native ISO. This may be useful if you wish to use the SxS recordings as a proxy file for the edit and would like the proxy files to look similar to the way the final footage will look after grading and correction for the EI offset.

WHAT IF YOU ARE SHOOTING USING HFR (High Frame Rate) AND LUT’S CANT BE USED.

In HFR you can either have LUT’s on for everything including internal recording, or all off, not LUT at all. This is not helpful if your primary recordings are internal SLog.

So if you can’t use the LUT’s you can use the VF High Contrast mode. Sadly this is only available in the viewfinder, but I find that it is much more obvious if your exposure is off when you use the VF High Contrast mode.

The VF High Contrast Mode acts as a 709(800) LUT for the viewfinder only. So expose at the native ISO, by eye, using normal 709 type levels and your Slog-3 should be pretty close to perfect.

The camera automatically turns this mode OFF when you power the camera down, so you must re-enable it when you power cycle the camera. This is probably a good thing as it means you shouldn’t accidentally have it turned on.

Sadly zebras etc either measure the LUT output or the Slog, they are NOT effected by the viewfinder HC mode, so in HFR they will be measuring the SLog. Also if the LUT’s are off then you can’t use different EI gains.

CINE-EI SUMMARY:

CineEI allows you to “rate” the camera at different ISO.

You MUST use a LUT for CineEI to work as designed.

A low EI number will result in a brighter exposure which will improve the signal to noise ratio giving a cleaner picture or allow you to see more shadow detail. However you will loose some over exposure headroom.

A high EI number will result in a darker exposure which will improve the over exposure headroom but decrease the under exposure range. The signal to noise ratio is worse so the final picture may end up with more noise.

A 1D LUT will not clip and appear to overexpose as readily as a 3D LOOK when using a low EI, so a 1D LUT may be preferable.

When viewing via a 709 LUT you expose using normal 709 exposure levels. Basically if it looks right in the viewfinder or on the monitor (via the 709 LUT) it almost certainly is right.

When I shoot with my F5 I normally rate the camera as 800EI. I find that 5 stops of over exposure range is plenty for most situations and I prefer the decrease in noise in the final pictures. I rate the F55 similarly at 640EI. But please, test and experiment for yourself.

QUICK GUIDE TO CREATING YOUR OWN LOOK’s (Using DaVinci Resolve).

It’s very easy to create your own 3D LUT for the Sony PMW-F5 or PMW-F55 using DaVinci Resolve or just about any grading software with LUT export capability. The LUT should be a 17x17x17 or 33x33x33 .cube LUT. This is what Resolve creates by default and .cube LUT’s are the most common types of LUT in use today.

First simply shoot some test Slog3 clips at the cameras native ISO. You must use Slog3 if you want to use User 3D LOOK’s in the camera. In addition you should also use the same color space for the test shot as you will when you want to use the LUT. I recommend shooting a variety of clips so that you can asses how the LUT will work in different lighting situations.

Import and grade the clips from the test shoot in Resolve creating the look that you are after for your production or as you wish your footage to appear in the viewfinder of the camera. Then once your happy with the look of the graded clip, right click on the clip in the timeline and “Export LUT”. Resolve will then create and save a .cube LUT.

Then place the .cube LUT file created by the grading software on an SD card in the PMWF55_F5 folder. You may need to create the following folder structure on the SD card. So first you have a PRIVATE folder, in that there is a SONY folder and so on.

PRIVATE   :   SONY   :    PRO   :   CAMERA   :    PMWF55_F5

Put the SD card in the camera, then go to the “File” menu and go to “Monitor 3D LUT” and select “Load SD Card”. The camera will offer you a 1 to 4 destination memory selection, choose 1,2,3 or 4, this is the location where the LUT will be saved. You should then be presented with a list of all the LUT’s on the SD card. Select your chosen LUT to save it from the SD card to the camera.

Once loaded in to the camera when you choose 3D User LUT’s you can select between user LUT memory 1,2,3 or 4. Your LUT will be in the memory you selected when you copied the LUT from the SD card to the camera.

Why gain is bad for your dynamic range.

One way to reduce the noise in a video camera image is to reduce the cameras gain. One way to increase the brightness of the image is to add gain.

We all know that increasing the gain to lets say +6db will increase noise and generally the reverse holds true when you reduce the gain, the noise typically reduces and this may be helpful if you are going to do a lot of effects work, or just want a clean image.

However in most cases adding or removing gain reduces the cameras dynamic range as it will artificially clip or limit your low key or high key parts of the image. The maximum illumination level that a camera can capture is limited by the sensor or the gamma curves that the camera has. The black level or darkest part of the image is the point where the actual image signal compared to the sensor noise level is high enough to allow you to see some actual picture information (also known as noise floor). So the dynamic range of the camera is normally the range between the sensors noise floor and recording or sensor clipping point.

To maximise the cameras dynamic range the designers will have carefully set the nominal zero db gain point (native ISO) so that the noise floor is at or very close to black and the peak recording level is reached at the point where the sensor itself starts to clip.

The gain of the camera controls the video output and recording level, relative to the sensors signal level. If you use -3db gain you attenuate (reduce) the relative output signal. The highlight handling doesn’t change (governed by the sensor clipping or gamma curve mapping) but your entire image output level gets shifted down in brightness and as a result you will clip off or loose some of your shadow and dark information, so your overall dynamic range is also reduced as you can’t “see” so far into the shadows. Dynamic range is not just highlight handling, it is the entire range from dark to light. 3db is half a stop (6db = 1 stop) so -3db gain reduces the dynamic range by half a stop, reducing the cameras underexposure range without (in most cases) any change to the over exposure range, so overall the total dynamic range is reduced.

When you add gain the reverse happens. Generally how far the sensor can see into the shadows is limited by the sensors noise floor. Add 6db of gain and you will make the darkest parts of the image brighter by 6db, but you will also raise the noise level by the same amount. So while you do end up with brighter shadow details you can’t actually see any more picture information because the noise level has increased by the same amount. At the top end as the brightest sensor output is mapped to the maximum recording level at 0db, when you add gain this pushes the recording level beyond what can be recorded, so you loose 6db off the top end of your recordings because the recordings and output clips 6db earlier. So positive gain maintains the same shadow range but reduces the highlight recording range by 6db.

However you use it gain tends to reduce your dynamic range. Adding gain to cope with poor lighting tends to be the lesser of the two evils as generally if your struggling for light then overexposure and blown out highlights is often the last of your worries.

Negative gain is sometimes used in camera to try to reduce noise, but the reality is that you are loosing dynamic range. Really a better solution would be to expose just a tiny bit brighter and then bring your levels down a bit in post production.

What’s the difference between Latitude and Dynamic Range?

These two words, latitude and dynamic range are often confused and are often used interchangeably.  Sometimes they can be the same thing (although rare), sometimes they may be completely different. So what is the difference and why do you have to be careful to use the right term.

Lets start with dynamic range as this is the simplest to understand. When talking about a digital camera the dynamic range is quite simply the total range from the darkest shadow to the brightest highlight that the camera can resolve in a single shot. To be included in the dynamic range you must be able to discern visually or measure with a scope a brightness change at both ends of the range. So a camera that can resolve 14 stops will be able to shoot a scene with a 14 stop brightness range and show some information from stop 0 to stop 14. It is not just a measure of the cameras highlight handling, it includes both highlights and shadows. One camera may be very low noise, so see very far into the shadows but not be so good with highlights. While another may be noisy, so not able to see so far into the shadows but have excellent highlight handling. Despite these differences both might have the same dynamic range as it is the range we are looking at, not just one end or the other.

One note of caution with published dynamic range figures or measurements is that while you may be able to discern some picture information in those deepest shadows or brightest highlights, just how useable both ends of the range are will depend on just how the camera performs at it’s extremes. It is not uncommon for the darkest stop to be so close to the cameras noise floor that in reality it’s barely useable, but as it can be measured it will be included in the manufacturers dynamic range figures.

This brings us on to latitude because latitude is a measure of just how flexible you can be with your exposure without significantly compromising the finished picture. The latitude will always be less than the cameras dynamic range. With a film camera, the film stock would have a sensitivity value or ISO. You would then use an exposure meter to determine the optimum exposure. The latitude would then be how much can you over expose or under expose and still have an acceptable result. But what is “an acceptable result”? Here is one of the key problems with determining latitude, what some people may find unacceptable others may be happy with so it can be difficult to quantify the exact latitude of a film stock or video camera precisely. However what you can do is determine which cameras have bigger ranges for example camera “A” has a stop more latitude than camera “B” provide you use a consistent “acceptable quality” assesment.

Anyone that’s shot with a traditional ENG or home video camera will know that you really need to get your exposure right to get a decent looking picture. Take a simple interview shot, expose it right and it looks fine. Overexpose by 1 stop and it looks bad, even if you try to grade it it will still look bad. So in this example the camera would have less than 1 stop of over exposure latitude. But if you underexpose a video camera, the picture gets darker, but after a bit of work in post production it may well still look OK. It will depend in most cases on how noisy the picture becomes when you boost the levels in post to brighten the picture. But typically you might be able to go 1 to 1.5 stops under exposed and still have a useable image. So in this case the camera would have 1.5 stops of underexposure latitude. This then gives a total latitude for our hypothetical camera of around 2 to 2.5 stops.

But what of we increase the dynamic range of the camera or have a camera with a very big dynamic range. Does my latitude increase?

Well the answer is maybe. In some cases the latitude may actually decrease. How can that be possible, surely with a bigger dynamic range my latitude must be greater?

Well, unless your shooting linear raw (more on that in a bit) you will be using some kind of gamma curve. The gamma curve is there to allow you to squeeze a large dynamic range into a small amount of data. It does this by mimicking the way we perceive light in a non linear manner and uses less data in highlights which are perceptually less important to us humans. Even uncompressed video normally has a gamma curve. Without a gamma curve the amount of data needed to record a decent looking picture would be huge as every additional stop of dynamic range actually needs twice as much data as the previous to be recorded faithfully.

With cameras with larger dynamic ranges then things such as knee compression or special gamma curves like Hypergamma, Cinegamma or Log are used. The critical thing with all of these is that the only way to squeeze that greater dynamic range into the same size recording bucket is by adding extra compression to the recorded image.

exposure1This compression is normally restricted to the highlights (which are perceptually less important). Highlight compression now presents us with an exposure problem, because if we over expose the shot then the picture won’t look good due to the compression. This means that even though we might have increased the cameras dynamic range (by squeezing and compressing more information into the highlight range) we may have reduced the exposure latitude as any over exposure places important mid range information into the highly compressed part of the gamma curve. So bigger dynamic range does not mean greater latitude, in fact in many cases it means less latitude.

Here’s the thing. Unless you make the recording data bucket significantly bigger (better codec and more bits, 10 bit 12 bit etc), you can’t put more data (dynamic range or stops) into that bucket without it overflowing or without squashing it. Given that most cameras used fixed 8 bit or 10 bit recording there is a finite limit to what can be squeezed into the codec without making some pretty big compromises.

Compression point with Hypergamma/Cinegamma.
Compression point with Hypergamma/Cinegamma.

With a standard gamma curve white is exposed around 90% to 95%, remember a white card only reflects 90% of the light falling on it not 100%. Middle grey perceptually appears half way between black and white so it’s around 40%-45%. Above 90% is where the knee normally acts to compress highlights to squeeze quite a large dynamic range into a very small recording range, so anything above 90% will be very highly compressed, but below 90% we are OK and we can safely use the full range up to 90%. Expose a face below 90% and it will look natural, above 90% it will look washed out, low contrast and generally nasty due to the squeezing together of the contrast and dynamic range.

But what about a Hypergamma or Cinegamma (or any other high dynamic range gamma curve)? Well these don’t have a knee, instead they start to gradually introduce compression much lower down the gamma curve. A little bit at first and then ever increasing amounts as we go up the exposure range. This allows them to squeeze in a much greater dynamic range in a pleasing way (provided you expose right). But this means that we can’t afford to let faces etc go as high as with the standard gamma because if we do they will start to creep in to the highly compressed part of the curve. So this means that even the slightest over exposure will hurt our image.  So even thought they have greater dynamic range, these curves have less exposure latitude because we really really can’t afford to over expose them. Sony compensate for this to some degree by recommending a lower middle grey point between 32 and 40% depending on the curve you use. This then brings your overall exposure lower so your less likely to over expose, but that now means you have less under exposure range as your already shooting a bit darker (White with the hypergammas tends to fall lower, around 80%, so faces and skin tones that would normally be around 70% will be around 60%).

More highlight compression means exposure is still critical despite greater dynamic range
More highlight compression means exposure is still critical despite greater dynamic range

But what about Log?

Now lets look at S-Log2, S-log3. Most  log curves are also similar, very highly compressed gamma curves with huge amounts of highlight compression to squeeze in an exceptionally large dynamic range. With Slog2 White is designed to be at 59% and middle grey at 32% and with S-log3 middle grey is 41% and white 61%. So faces will need to sit between around 40% and 50% to look their best. Now log is a little bit different. Log shooting is designed to be done in conjunction with LUT’s (Look Up Tables) in post production. These LUT’s convert the signal from Log gamma to conventional gamma. When you apply the correct LUT to correctly exposed Log everything comes out looking good. What about over exposed Log? This is where it can get tricky. If you have a good exposure correction LUT or really know how to grade log properly (which can be tricky) then you can expose Log by one or 2 stops, but no more (in my opinion at least, 2 stops is a lot of over exposure for Log, I would try to stay less than 2 stops over). Over expose too much and the image gets really hard to grade and may start to lack contrast. One thing to note is when I say over-exposed with respect to log, I’m not talking about about a clipped picture, but simply an image much brighter than it should be. For example with Slog3 faces will be around 52%. If you expose faces at 70% your actually just over 2 stops over exposed and grading is going to start to get tricky and you may find it hard to get your skin tones just right. So, when shooting log make sure you know what the recommended levels are for the curve you are using. I’m not saying you can’t over expose a bit, just be aware of what is correct and that level shifts of just a 7 or 8% may represent a whole stop of exposure change.

It’s only when you stop shooting with conventional gamma curves and start shooting linear that the latitude really starts to open up. Cameras like the Sony F5/F55 use linear raw recording that does not have a gamma curve. When you have no gamma curve then there is no highlight compression. So for example you could expose a face anywhere between in conventional terms between say 45% (the point where perhaps it becomes too noisy if you expose any darker) and 100% it will look just fine after grading because at no point does it become compressed. This is a massive latitude increase over a camera using a gamma curve. It gets even better if the camera is very low noise as you can afford to expose at an even lower level and bring it up in post. This is why raw is such a big deal. I find it much easier to work with and grade raw than log because raw just behaves nicely.

In Conclusion:

Dynamic range is the range the camera can see from the deepest darkest shadows to the brightest highlights in the same shot. Latitude is the range within the dynamic range where we can expose and still get a useable image.

A camera with lower noise will allow you to expose darker and bring your levels up in post, this gives an increase in under exposure range.

Most video cameras have a very limited over exposure latitude due to aggressive highlight compression. This is the opposite to a film camera.

Bigger dynamic range does not always mean greater latitude.

Cameras that shoot raw typically have a much greater latitude than a camera shooting with a gamma curve. For example an F5 shooting SLog2/3 has a much smaller exposure latitude than when shooting raw even though the dynamic range is the same in both cases.

 

S-Log, Latitude, Dynamic Range and EI S-log. Or how to modify your exposure range with EI S-Log

The big issue most people have when working with log and exposing mid grey at 38 is that when you look at it on a standard monitor without any lookup tables it looks underexposed. The assumption therefore is that it is underexposed or in some way too dark to ever look right, because that’s what people used to working with conventional gammas have become programmed to believe over many years from their experience with conventional gammas.

So, for confidence you add a lookup table which converts the log to a Rec-709 type gamma and now the image looks brighter, but as it now has to fit within Rec-709 space we have lost either some of our high end or low end so we are no longer seeing the full range of the captured image so highlights may be blown out or blacks may be crushed.
It’s important for people to understand the concept of gamma and colour space and how the only way to truly see what a camera (any camera) is capturing is to use a monitor that has the same gamma and colour space. Generally speaking lookup tables don’t help as they will be taking a signal with a large range and manipulating it to fit in a small range and when you do that, something has to be discarded. If you were to take an F3 set to S-log and expose mid grey at 38 and show that on one of the nice new Sony E170 series monitors that have S-log gamma and place that next to another F3 with Rec-709 shooting mig grey at 45% and a similar but conventional 709 monitor the lower and mid range exposures would be near identical and the S-log images would not look under exposed or flat. The S-log images however would show an extra 2 stops of dynamic range.

Furthermore it has to be remembered that log is log, it is not linear. Because of its non linear nature, less and less brightness information is getting recorded as you go up the brightness range. As our own visual system is tuned to be most accute in the mid ranges this is normally fine provide you expose correctly putting mid tones in the more linear, lower parts of the S-log curve. Start putting faces to high up the S-log curve and it gets progressively harder to get a natural look after grading. This is where I think a lot of people new to log stumble. They don’t have the confidence to expose faces at what looks like a couple of stops under where they would with a standard gamma, so they start bringing up the exposure closer to where they would with standard gamma and then have a really hard time getting faces to look natural in the grade. Remember that the nominal S-Log value for white is 68 IRE. Part of the reason for this is that above about 70 IRE the amount of compression being applied by log is getting pretty extreme. While there is some wriggle room to push your exposure above or below the nominal mid grey at 38 it’s not as big as you might expect, especially dealing with natural tones and overexposure.

If you do want to shift your middle grey point this is where the EI S-log function and a light meter comes into it’s own, it’s what it’s designed for.

First something to understand about conventional camera gain, dynamic range and latitude. The latitude and sensitivity of the F3 is governed by the latitude and sensitivity of the sensor, which is a little under 13 stops. Different amounts of gain or different ISO’s don’t alter the sensors latitude, nor do they alter the actual sensitivity, only the amount of signal amplification. Increasing the camera gain will reduce the cameras output dynamic range as something that is 100 IRE at 800 ISO would go into clipping if the actual camera gain was increased by 6db (taking the ISO to 1600) but the darkest object the camera can actually detect remains the same. Dark objects may appear brighter, but there is still a finite limit to how dark an object the camera can actually see and this is governed by the sensor and the sensors noise floor.

EI (Exposure Index) shooting works differently, whether it’s with the F3, F65, Red or Alexa. Let’s consider how it works with the PMW-F3. In EI S-Log mode the camera always actually outputs at 800 ISO from the A/B outputs. It is assumed that if your working with S-Log you will be recording using an external 10 bit recorder connected to the A/B outputs. 422 is OK, but you really, really need 10 bit for EI S-Log. At 800 ISO you have 6.5 stops of over exposure and 6.5 under when you shoot mid grey at 38 or expose conventionally with a light meter.
Now what happens when you set the camera to EI 1600? Understand that the camera will still output at 800 ISO over the A/B outputs to your external recorder, but also note that 6db gain (1 stop) is added to the monitor output and what you see on the LCD screen, so the monitor out and LCD image get brighter. As the cameras metering systems (zebras, spot meter, histogram) measure the signal on the monitor side these are also now offset by +6db or + 1 stop.
As the camera is set to EI 1600 we set our light meter to 1600 ISO. If we make no change to our lighting the light meter would tell us to stop down by one stop, compared to our original 800 ISO exposure.
Alternately, looking at the camera, when you switch on EI 1600 the picture gets brighter, your mid grey card would also become brighter by one stop, so If we use the cameras spot meter to expose our grey card at 38 again we would need to stop down the iris by one stop to return the grey card to 38 IRE (for the same light levels as we used for 800). So either way, whether exposing with a light meter or exposing using the cameras built in metering, when you go from EI 800 to EI 1600 for the correct exposure (under the same lighting) you would stop down the iris by one stop.
Hope those new to this are still with me at this point!
Because the cameras A/B output is still operating at 800 ISO and you have stopped down by one stop as that what the light meter or camera metering told you to do because they are operating at EI 1600, the A/B output gets darker by one stop. Because you have shifted the actual recorded output down by one stop you have altered you exposure range from the original +/- 6.5 stops to + 7.5 stops, -5.5 stops. So you can see that when working at EI 1600 the dynamic range now becomes + 7.5 stops and -5.5 stops. Go to EI 3200 and the dynamic range becomes +8.5 stops and -4.5 stops.
So EI S-log gives you a great way of shifting your dynamic range centre while giving you consistent looking exposure and a reasonable approximation of how your noise levels are changing as you shift your exposure up and down within the cameras dynamic range.
EI S-Log doesn’t go below 800 because shifting the dynamic range up the exposure range is less beneficial. Lets pretend you have an EI 400 setting. If you did use it, you would be opening up the iris by one stop, so your range becomes +5.5 and -7.5 stops compared to your mid grey or light metered exposure. So you are working with reduced headroom and you are pushing your mid range up into the more highly compressed part of the curve which is less desirable. I believe this is why the option is not given on the F3.

Setting Exposure with Standard Gammas – Use your judgement!

First take a long look at the image to the left. look at the 3 small grey boxes. What do you see?
Is the middle of the three grey boxes brighter than the others? Does the bottom small grey box look about the same brightness as the top one?

This post comes as the result of a discussion going on elsewhere about how to correctly expose when using standard gammas. Basically discussing how to expose when your not going to do anything to your footage in post,  for what I would call “direct to air”.
There are many ways of setting exposure. You could use a light meter, you could use zebras,  you could use a waveform monitor or histogram.

Lets imagine that grey box is a face. If you were using zebras you would normally set them to between 65% and 70% and then expose the shot so the face exhibited the zebra pattern over any parts of the face not overly highlighted or in shadow. This is the textbook way to expose using zebras. Another way to expose might be to use a mid grey card (also known as an 18% grey card). With standard and cinegammas you would normally expose this at 50% using the cameras histogram, waveform monitor or spot meter. Again this is a textbook, technically correct exposure. But this is the real world and the real world is very different to the theoretical world because light plays tricks with our eyes and the overall brightness of a scene can change the mood of the shot.

Lets say you have a room with dark coloured walls.  At one end is a window and you have an actor standing at each end of the room, one against the dark wall, one against the window. We have two shots in our scene, one looking at the actor against the dark wall, one looking at the actor against the window. What happens if we expose both faces using zebras to exactly the same textbook 65% level? Well the face against the window will look darker than the face against the black wall. Look back at the grey boxes on the left. The top and middle grey boxes are exactly the same brightness but because the middle box is against black, to our eye’s it appears brighter than the top one. Now if we were to use a histogram or waveform monitor to expose these two shots, all the extra white in the window shot might tempt you to reduce the exposure, this would make the problem even worse. In fact to expose these two shots so that the faces match as you cut between them you need to reduce the exposure on the darker shot. Looking at the grey boxes again the lowest box is actually at 45% while the other two are at 65%, yet the lower box appears to be about as bright as the top box.
So what am I trying to say? Well exposure isn’t all about setting object “X” at exposure “Y”. You must use your judgement and a known monitor or viewfinder to asses your pictures. Learn to interpret what your monitor is telling you, learn to recognise scenes that may need to be exposed away from the text book values and methods. Above all else don’t be afraid to expose for what looks right, as opposed to object “X” at value “Y”.
I suppose to follow up on this I should tell you how to calibrate your viewfinder or monitor… I’ll do that soon in a later article. Did you find this useful? let me know, I’m planning on writing more about dealing with light and lighting.

Exposing when shooting S-Log.

The question over whether to deliberately underexpose or not with S-Log came up recently. I believe that you need to evaluate the entire scene when shooting S-Log and that the often heard “underexpose by a stop” methodology may have some issues. Here’s my take on the situation:

A couple of caveat’s first: Most of my F3 S-Log work has been in indoor situations as I have been tied to recording to various less than portable 10 bit recording solutions, so very often using a restricted contrast range. I’ve only owned S-Log for my F3’s for a short while now, so many of my earlier tests were on 3rd party cameras, some of these were beta cameras.

I have not fully tied down my workflow. I’m still investigating external recorders, everything from the Ninja, Ki-Pro, Sound Devices and of course Gemini. I’m leaning very heavily towards the Gemini as I do a lot of 3D and the Gemini LCD makes for a fantastic monitor.
Back to exposure, this is obviously going to be a slightly contentious area as there is no real “correct way to do it”. While I might not agree with pinning skin tones or anything else for that matter to one particular brightness range, that does not mean I’m right and anyone else is wrong, it is just a different approach and methodology. At the end of the day, if it works for you and gets the results you want, then that will be the way you should go, these things are not black and white, right or wrong.
A very un-scientific test that a did a while back was an eye opener for me. I was exploring the finite latitude of S-Log compared to the F3’s cinegammas. I did a couple of very quick shots, you will find them here: http://www.xdcam-user.com/2011/06/pmw-f3-s-log-and-cinegamma-quick-look/
When I filmed these two examples I was looking at dynamic range, I exposed in both cases with the bright whites of the back wall behind the girl just going into clipping so I could then see how far into the shadows I could still see useable detail. I was not concerned about getting the skin tone exposure correct. When you look at the raw S-Log it really looks pretty shocking and even I wasn’t sure how much I would recover from the highlights and the girl is a good stop overexposed. However after a very simple grade using only the colour corrector in FCP, I was able to extract a pretty good looking image and it’s amazing how much detail was actually retained in what looked like over exposed high lights.  The Girl’s skin tones which I’ve measured at over 85IRE came down very nicely without any issue. A proper grade in a grading suite would I’m sure improve them still further.
What this very crude test told me was that you have incredible flexibility over where you put skin tones, you can comfortably move them up and down in post by a quite significant margin. Also seemingly overexposed S-Log highlights will contain surprisingly large amounts of fully recoverable detail. In the same test I graded the Cinegamma material to try to recover the shadow detail that was lost by due to the reduced latitude. This involved attempting to pull up the shadow areas. While this was somewhat successful, what became very apparent was the way the noise increased quite dramatically, this is something I have been aware of since I started using Cinegammas many years ago, pulling levels up will increase noise.
So… when I expose with Cinegammas (as I have done for many years) I have always been very conscious of the noticeable effect on noise that trying to lift underexposed parts of the image has. Very often in the grade the limiting factor as to how far you can push the image has been down to the noise floor and noise effects. This has mainly been with Sony EX’s which have a 54db noise floor.
Now with the F3 with have a dilemma! S-Log gives us another +1.5ish stops of dynamic range, but at the expense of a +6db increase in noise due to the +1 stop increase in sensitivity associated with S-Log.
Lets say for example that we shoot a shot with a person and we under expose the face by one stop (one stop = 6db).
If we do this with with the Cinegammas and then grade the shot bringing the face up one stop then the noise will increase by 6db from the base noise figure of 63db giving a final noise figure of approx 57db (in the case of signal to noise, a lower number is worse).
If we do this with S-Log and then grade the shot bringing up the face by one stop then the noise will increase by 6db from the base of 57db giving a final noise figure of approx 51db.
So the S-Log image becomes twice as noisy as the cinegamma material and therefore depending on the footage, it is quite possible that you would actually be able to push mid ranges and shadows further with Cinegammas than S-log in an underexposed situation due to noise issues. The S-Log and Cinegamma curves are almost identical up to over 50IRE, so latitude performance under 50IRE is essentially the same. See the charts on this page: http://www.xdcam-user.com/2011/05/s-log-a-further-in-depth-look/
If I get some time at IBC I might see if I can set up some tests to show this in practice.
Now given that I have seen for myself how with S-Log skin tones can be pushed down just as much as up in post, I tend to try to evaluate the entire scene and consider how it will be treated in post before choosing how to expose. In particular I don’t want to expose so that the entire scene will end up being lifted by a significant amount, as noise will become a concern. This isn’t always going to be possible as there are many shots where highlights have to be protected, but I don’t believe that you have to set skins etc at any particular narrow brightness range, I tend to let skin ride somewhere between 45IRE and 70IRE depending on the overall scene.
If I can fit the contrast range of the scene into the 11.5 stops of a cinegamma then I will often use the cinegammas over S-Log because of the noise improvement. S-Log comes into it’s own where you have an extreme contrast range that needs to be captured. However at the end of the day you do still have to remember that the end display device is unlikely to be able to display more than 7 stops with any accuracy!
One tool I have found very useful is the BlackMagic HDLink box. I often use this to connect to a monitor as it has the ability to apply LUT’s very quickly. If you have a PC connected to the HDLink you can go in an modify the LUT curve in real time and in effect do an on-set grade. The HDLink is only $499 USD.