Tag Archives: gamma

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

August 19, 2014 alisterchapman 39 Comments

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.

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. 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.

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.

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.

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.

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:

As 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!

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.

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.

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.

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.

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.

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.

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.

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.

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

July 14, 2014 alisterchapman Leave a comment

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/

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What is a Gamut or Color Space and why do I need to know about it?

May 6, 2014 alisterchapman 6 Comments

Well I have set myself quite a challenge here as this is a tough one to describe and explain. Not so much perhaps because it’s difficult, but just because it’s hard to visualise, as you will see.

First of all the dictionary definition of Gamut is “The complete range or scope of something”.

In video terms what it means is normally the full range of colours and brightness that can be either captured or displayed.

I’m sure you have probably heard of the specification REC-709 before. Well REC-709, short for ITU-R Recommendation, Broadcast Television, number 709. This recommendation sets out the display of colours and brightness that a television set or monitor should be able to display. Note that it is a recommendation for display devices, not for cameras, it is a “display reference” and you might hear me talking about when things are “display referenced” ie meeting these display standards or “scene referenced” which would me shooting the light and colours in a scene as they really are, rather than what they will look like on a display.

Anyway…. Perhaps you have seen a chart or diagram that looks like the one below before.

Now this shows several things. The big outer oval shape is what is considered to be the equivalent to what we can see with our own eyes. Within that range are triangles that represent the boundaries of different colour gamuts or colour ranges. The grey coloured triangle for example is REC-709.

Something useful to know is that the 3 corners of each of the triangles are whats referred to as the “primaries”. You will hear this term a lot when people talk about colour spaces because if you know where the primaries (corners) are, by joining them together you can find the size of the colour space or Gamut and what the colour response will be.

Look closely at the chart. Look at the shades of red, green or blue shown at the primaries for the REC-709 triangle. Now compare these with the shades shown at the primaries for the much larger F65 and F55 primaries. Is there much difference? Well no, not really. Can you figure out why there is so little difference?

Think about it for a moment, what type of display device are you looking at this chart on? It’s most likely a computer display of some kind and the Gamut of most computer displays is the same size as that of REC-709. So given that the display device your looking at the chart on can’t actually show any of the extended colours outside of the grey triangle anyway, is it really any surprise that you can’t see much of a difference between the 709 primaries and the F65 and F55 primaries. That’s the problems with charts like this, they don’t really tell you everything that’s going on. It does however tell us some things. Lets have a look at another chart:

This chart is similar to the first one we looked at, but without the pretty colours. Blue is bottom left, Red is to the right and green top left.

What we are interested in here is the relationship between the different colour space triangles. Using the REC-709 triangle as our reference (as that’s the type of display most TV and video productions will be shown on) look at how S-Gamut and S-Gamut3 is much larger than 709. So S-Gamut will be able to record deeper, richer colours than 709 can ever hope to show. In addition, also note how S-Gamut isn’t just a bigger triangle, but it’s also twisted and distorted relative to 709. This is really important.

You may also want to refer to the top diagram as well as I do my best to explain this. The center of the overall gamut is white. As you draw a line out from the center towards the colour spaces primary the colour becomes more saturated (vivid). The position of the primary determines the exact hue or tone represented. Lets just consider green for the moment and lets pretend we are shooting a shot with 3 green apples. These apples have different amounts of green. The most vivid of the 3 apples has 8/10ths of what we can possibly see, the middle one 6/10ths and the least colourful one 4/10ths. The image below represents what the apples would look like to us if we saw them with our eyes.

The apples as we would see them with our own eyes.

If we were shooting with a camera designed to match the 709 display specification, which is often a good idea as we want the colours to look right on the TV, the the greenest, deepest green we can capture is the 709 green primary. lets consider the 709 green primary to be 6/10ths with 10/10ths being the greenest thing a human being can see. 6/10ths green will be recorded at our peak green recording level so that when we play back on a 709 TV it will display the greenest the most intense green that the display panel is capable of. So if we shoot the apples with a 709 compatible camera, 6/10ths green will be recorded at 100% as this is the richest green we can record (these are not real levels, I’m just using them to illustrate the principles involved) and this below is what the apples would look like on the TV screen.

6/10ths Green and above recorded at 100% (our imaginary rec-709)

So that’s rec-709, our 6/10ths green apple recorded at 100%. Everything above 6/10 will also be 100% so the 8/10th and 6/10ths green apples will look more or less the same.

What happens then if we record with a bigger Gamut. Lets say that the green primary for S-Gamut is 8/10ths of visible green. Now when recording this more vibrant 8/10ths green in S-Gamut it will be recorded at 100% because this is the most vibrant green that S-Gamut can record and everything less than 8/10 will be recorded at a lower percentage.

But what happens if we play back S-Gamut on a 709 display? Well when the 709 display sees that 100% signal it will show 6/10ths green, a paler less vibrant shade of green than the 8/10ths shade the camera captured because 6/10ths is the most vibrant green the display is capable of. All of our colours will be paler and less rich than they should be.

The apples recorded using a big gamut but displayed using 709 gamut.

So that’s the first issue when shooting with a larger colour Gamut than the Gamut of the display device, the saturation will be incorrect, a dark green apple will be pale green. OK, that doesn’t sound like too big a problem, why don’t we just boost the saturation of the image in post production? Well if the display is already showing our 100% green S-Gamut signal at the maximum it can show (6/10ths for Rec-709) then boosting the saturation won’t help colours that are already at the limit of what the display can show simply because it isn’t capable of showing them any greener than they already look. Boosting the saturation will make those colours not at the limit of the display technology richer, but those already at the limit won’t get any more colourful. So as we boost the saturation any pale green apples become greener while the deep green apples stay the same so we loose colour contrast between the pale and deep green apples. The end result is an image that doesn’t really look any different that it would have done if shot in Rec-709.

Saturation boosted S-Gamut looks little different to 709 original.

But, it’s even worse that just a difference to the saturation. Look at the triangles again and compare 709 with S-Gamut. Look at how much more green there is within the S-Gamut colour soace than the 709 colour space compared to red or blue. So what do you think will happen if we try to take that S-Gamut range and squeeze it in to the 709 range? Well there will be a distinct colour shift towards green as we have a greater percentage of green in S-Gamut than we should have in Rec-709 and that will generate a noticeable colour shift and the skewing of colours.

Squeezing S-Gamut into 709 will result in a colour shift.

This is where Sony have been very clever with S-Gamut3. If you do take S-Gamut and squeeze it in to 709 then you will see a colour shift (as well as the saturation shift discussed earlier). But with S-Gamut3 Sony have altered the colour sampling within the colour space so that there is a better match between 709 and S-Gamut3. This means that when you squeeze S-Gamut3 into 709 there is virtually no colour shift. However S-Gamut3 is still a very big colour space so to correctly use it in a 709 environment you really need to use a Look Up Table (LUT) to re-map it into the smaller space without an appreciable saturation loss, mapping the colours in such a way that a dark green apple will still look darker green than a light green apple but keeping within the boundaries of what a 709 display can show.

Taking this one step further, realising that there are very few, if any display devices that can actually show a gamut as large as S-Gamut or S-Gamut3, Sony have developed a smaller Gamut known as S-Gamut3.cine that is a subset of S-Gamut3.

The benefit of this smaller gamut is that the red green and blue ratios are very close to 709. If you look at the triangles you can see that S-Gamut3.cine is more or less just a larger version of the 709 triangle. This means that colours shifts are almost totally eliminated making this gammut much easier to work with in post production. It’s still a large gamut, bigger than the DCI-P3 specification for digital cinema, so it still has a bigger colour range than we can ever normally hope to see, but as it is better aligned to both P3 and rec-709 colourists will find it much easier to work with. For productions that will end up as DCI-P3 a slight saturation boost is all that will be needed in many cases.

So as you can see, having a huge Gamut may not always be beneficial as often we don’t have any way to show it and simply adding more saturation to a seemingly de-saturated big gamut image may actually reduce the colour contrast as our already fully saturated objects, limited by what a 709 display can show, can’t get any more saturated. In addition a gamut such as S-Gamut that has a very different ratio of R, G and B to that of 709 will introduce colour shifts if it isn’t correctly re-mapped. This is why Sony developed S-Gamut3.cine, a big but not excessively large colour space that lines up well with both DCI-P3 and Rec-709 and is thus easier to handle in post production.

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What’s the difference between Latitude and Dynamic Range?

November 18, 2013 alisterchapman 2 Comments

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.

This 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.

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

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.

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Choosing the right gamma curve.

May 14, 2013 alisterchapman 1 Comment

One of the most common questions I get asked is “which gamma curve should I use?”.

Well it’s not an easy one to answer because it will depend on many things. There is no one-fits-all gamma curve. Different gamma curves offer different contrast and dynamic ranges.

So why not just use the gamma curve with the greatest dynamic range, maybe log? Log and S-Log are also gamma curves but even if you have Log or S-Log it’s not always going to be the best gamma to use. You see the problem is this: You have a limited size recording bucket into which you must fit all your data. Your data bucket, codec or recording medium will also effect your gamma choice.

If your shooting and recording with an 8 bit camera, anything that uses AVCHD or Mpeg 2 (including XDCAM), then you have 235 bits of data to record your signal. A 10 bit camera or 10 bit external recorder does a bit better with around 940 bits of data, but even so, it’s a limited size data bucket. The more dynamic range you try to record, the less data you will be using to record each stop. Lets take an 8 bit camera for example, try to record 8 stops and that’s about 30 bits per stop. Try to extend that dynamic range out to 11 stops and now you only have about 21 bits per stop. It’s not quite as simple as this as the more advanced gamma curves like hypergammas, cinegammas and S-Log all allocate more data to the mid range and less to highlights, but the greater the dynamic range you try to capture, the less recorded information there will be for each stop.

In a perfect world you would choose the gamma you use to match each scene you shoot. If shooting in a studio where you can control the lighting then it makes a lot of sense to use a standard gamma (no knee or knee off) with a range of up to 7 stops and then light your scene to suit. That way you are maximising the data per stop. Not only will this look good straight out of the camera, but it will also grade well provided your not over exposed.

However the real world is not always contained in a 7 stop range, so you often need to use a gamma with a greater dynamic range. If your going direct to air or will not be grading then the first consideration will be a standard gamma (Rec709 for HD) with a knee. The knee adds compression to just the highlights and extends the over-exposure range by up to 2 or 3 stops depending on the dynamic range of the camera. The problem with the knee is that because it’s either on or off, compressed or not compressed it can look quite electronic and it’s one of the dead giveaways of video over film.

If you don’t like the look of the knee yet still need a greater dynamic range, then there are the various extended range gammas like Cinegamma, Hypergamma or Cinestyle. These extend the dynamic range by compressing highlights, but unlike the knee, the amount of compression starts gradually and get progressively greater. This tends to look more film like than the on/off knee as it tends to roll off highlights much more gently. But, to get this gentle roll-off the compression starts lower in the exposure range so you have to be very careful not to over expose your mid-range as this can push faces and skin tones etc into the compressed part of the curve and things won’t look good. Another consideration is that as you are now moving away from the gamma used for display in most TV’s and monitors the pictures will be a little flat so a slight grade often helps with these extended gammas.

Finally we come to log gammas like S-Log, C-Log etc. These are a long way from display gamma, so will need to be graded to like right. In addition they are adding a lot of compression (log compression) to the image so exposure becomes super critical. Normally you’ll find the specified recording levels for middle grey and white to be much lower with log gammas than conventional gammas. White with S-Log for example should only be exposed at 68%. The reason for this is the extreme amount of mid to highlight compression, so your mid range needs to be recorded lower to keep it out of the heavily compressed part of the log gamma curve. Skin tones with log are often in the 40 – 50% range compared to the 60-70% range commonly used with standard gammas. Log curves do normally provide the very best dynamic range (apart from raw), but they will need grading and ideally you want to grade log footage in a dedicated grading package that supports log corrections. If you grade log in your edit suite using linear (normal gamma) effects your end results won’t be as good as they could be. The other thing with log is now your recording anything up to 13 or 14 stops of dynamic range. With an 8 bit codec that’s only 17 – 18 bits per stop, which really isn’t a lot, so for log really you want to be recording with a very high quality 10 bit codec and possibly an external recorder. Remember with a standard gamma your over 30 bits per stop, now were looking at almost half that with log!

Shooting flat: There is a lot of talk about shooting flat. Some of this comes from people that have seen high dynamic range images from cameras with S-Log or similar which do look very flat. You see, the bigger the captured dynamic range the flatter the images will look. Consider this: On a TV, with a camera with a 6 stop range, the brightest thing the camera can capture will appear as white and the darkest as black. There will be 5 stops between white and black. Now shoot the same scene with a camera with a 12 stop range and show it on the same TV. Again the brightest is white and black is black, but the original 6 stops that the first camera was able to capture are now only being shown using half of the available brightness range of the TV as the new camera is capturing 12 stops in total, so the first 6 stops will now have only half the maximum display contrast. The pictures would look flatter. If a camera truly has greater dynamic range then in general you will get a flatter looking image, but it’s also possible to get a flat looking picture by raising the black level or reducing the white level. In this case the picture looks flat, but in reality has no more dynamic range than the original. Be very careful of modified gammas said to give a flat look and greater dynamic range from cameras that otherwise don’t have great DR. Often these flat gammas don’t increase the true dynamic range, they just make a flat picture with raised blacks which results in less data being assigned to the mid range and as a result less pleasing finished images.

So the key points to consider are:

Where you can control your lighting, consider using standard gamma.

The bigger the dynamic range you try to capture, the less information per stop you will be recording.

The further you deviate from standard gamma, the more likely the need to grade the footage.

The bigger the dynamic range, the more compressed the gamma curve, the more critical accurate mid range exposure becomes.

Flat isn’t always better.

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To shoot flat or not to shoot flat?

March 31, 2013 alisterchapman 8 Comments

There is a lot of hype around shooting flat. Shooting flat has become a fashionable way to shoot and many individuals and companies have released camera settings said to provide the flattest images or to maximise the camera dynamic range. Don’t get me wrong, I’m not saying that shooting flat is necessarily wrong or that you shouldn’t shoot flat, but you do need to understand the compromises that can result from shooting flat.

First of all what is meant by shooting flat? The term comes from the fact that images shot flat look, err, well…. flat when viewed on a standard TV or monitor. They have low contrast and may often look milky or washed out. Why is this? Well most TV’s and monitors only have a contrast range that is the equivalent of about 7 stops. (Even a state of the art OLED monitor only has a range of about 10 to 11 stops). The whole way we broadcast and distribute video is based on this 7 stop range. The majority of HD TV’s and monitors use a gamma curve based on REC-709, which also only has a 6 to 7 stop range. Our own visual system has a dynamic range of up to 20 stops (there is a lot of debate over exactly how big the range really is and in bright light our dynamic range drops significantly). So we can see a bigger range than most TV’s can show, so we can see bright clouds in the sky as well as deep shadows while a TV would struggle to show the same scene.

Modern camera sensors have dynamic ranges larger than 7 stops, so we can capture a greater dynamic range than the average monitor can show. Now consider this carefully: If you capture a scene with a 6 stop range and then show that scene on a monitor with a 6 stop range, you will have a very true to life and accurate contrast range. You will have a great looking high contrast image. This is where having matching gammas in the camera and on the monitor comes in to play. Match the camera to the monitor and the pictures will look great, 7 stops in, 7 stops out. But, and it’s a big BUT. Real world scenes very often have a greater range than 6 or 7 stops.

A point to remember here: A TV or monitor has a limited brightness range. It can only ever display at it’s maximum brightness and best darkness. Trying to drive it harder with a bigger signal will not make it any brighter.

Feed the monitor with an image with a 7 stop range and the monitor will be showing it’s blackest blacks and it’s brightest whites.

But what happens if we simply feed a 7 stop monitor with an 11 stop image? Well it can’t produce a brighter picture so the brightest parts of the displayed scene are no brighter and the darker, no darker so the image you see appears to have the same brightness range but with less contrast, it starts to look flat and un-interesting. The bigger the dynamic range you try to show on your 7 stop monitor, the flatter the image will look. Clearly this is undesirable for direct TV broadcasting etc. So what is normally done is to map the first 5 stops from the camera more or less directly to the first 5 stops of the display so that the all important shadows and mid-tones have natural looking contrast. Then take the brighter extended range of the camera, which may be 3 or 4 stops and map those into the remaining 2 stops of the monitor. This is a form of compression. In most cases we don’t notice it as it is only effecting highlights and our own visual system tends to concentrate on shadows and mid-tones while largely ignoring highlights. This compression is achieved using techniques such as knee compression and is one of the things that gives video it’s distinctive electronic look.

A slightly different approach to just compressing the highlights is to compress much more of the cameras output. Gamma curves like Sony’s cinegammas or hypergammas use compression that gets progressively more aggressive as you go up the exposure range. This allows even greater dynamic ranges to be captured at the expense of a slight lack of contrast in the viewed image. Taking things to the maximum we have gamma curves that use log based compression where each brighter stop is compressed twice as much as the previous one. Log gamma curves like S-Log or Log-C are capable of capturing massive dynamic ranges of anywhere up to 14 stops. View these log compressed images back on your conventional TV or monitor and because even the mid range is highly compressed they will look very low contrast and very flat indeed.

So, if you have followed this article so far you should understand that we can capture a greater dynamic range than most monitors can display, but when doing so the image looks un-interesting and flat. So, if the images look bad, why do it? The benefits of capturing a big dynamic range are that highlights are less likely to look over exposed and your final image contrast can be adjusted in post production. These are the reasons why it is seen as desirable to shoot flat. But there are several catches. One is that the amount of image noise that the camera produces will limit how far you can manipulate your image in post production. The codec that you use to record your pictures may also limit how much you can manipulate your image, the bit depth of the codec may result in banding when stretched and another is that it is quite easy to create a camera profile or setup that produces a flat looking image, for example by artificially raising the shadows, that superficially looks like a flat, high dynamic range image, but doesn’t actually provide a greater dynamic range.

Of course there are different degrees of flat. There is super flat log style shooting as well as intermediate flat-ish cinegamma or hypergamma shooting. But it if you are going to shoot flat it is vital that the recorded image coming from the camera will stand up to the kind of post production manipulation you wish to apply to it. This is especially important when using highly compressed codecs such as AVCHD, XDCAM or P2.

When you use a high compression codec it adds noise to the image, this is in addition to any sensor noise etc. If you create a look in camera, the additional compression noise is added after the look has been created. As the look has been set, the compression noise is not really going to change as you won’t be making big changes to the image. But if you shoot flat, when you start manipulating the image the compression noise gets pushed, shoved and stretched, this can lead to degradation of the image compared to creating the look in camera. In addition you need more data to record a bigger dynamic range, so a very flat (wide dynamic range) image may be pushing the codec very hard resulting in even more compression noise and artefacts.

So if you do want to shoot flat you need a camera with very low noise. You also need a robust codec, preferably 10 bit and you need to ensure that the camera setup or gamma is truly capturing a greater dynamic range, otherwise your really wasting your time.

Shooting flat is a great tool in the cinematographers tool box and with the right equipment can bring great benefits in post production flexibility. Most of the modern large sensor cameras with their low noise sensors and ability to record to high end 10 bit codecs either internally or externally are excellent tools for shooting flat. But small sensor cameras with their higher noise levels do not make the best candidates for shooting flat. In many cases a better result will be obtained by creating your desired look in camera. Or at least getting close to the desired look in camera and then just tweaking and fine tuning the look in post.

As always, test your workflow. Just because so and so shoots flat with camera A, it doesn’t mean that you will get the same result with camera B. Shoot a test before committing to shooting flat on a project, especially if the camera isn’t specifically designed and set up for flat shooting. Shooting flat will not turn a poor cinematographer into a great cinematographer, in fact it may make it harder for a less experienced operator as hitting the cameras exposure sweet spot can be harder and focussing is trickier when you have a flat low contrast image.

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

June 11, 2012 alisterchapman 11 Comments

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.

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REC-709 – A true REC-709 camera should only have 5 stops of dynamic range!

April 26, 2012 alisterchapman 1 Comment

In theory if two cameras are both set to REC-709 then the dynamic range should be identical as both cameras responses will be limited to the REC-709 specifications. The original REC-709 specification only allows for 5 stops of dynamic range from 0 to 100%. The addition of superwhites at 109% gives a little more and then the knee on top gives a bit more again. The whole point behind REC-709 is that when a REC-709 camera captures something of a specific brightness, then when that image is shown on a TV or monitor, the apparent brightness will be exactly the same. As REC-709 is based on legacy television standards that go back to the very beginning of television broadcasting, it only allows for the dynamic range of older television sets. If a camera deviates from REC-709 then there will be a miss-match between the image the camera records and the image the viewer sees.

So, if we have two cameras with sensors capable of a dynamic range greater than the REC-709 specifications and then we restrict them to REC-709, they should exhibit identical dynamic range. Just like two cars fitted with speed limiters, they would both have identical maximum speeds. In fact the REC-709 specification is actually a viewing and display specification, not a camera specification and in reality the REC-709 gamma curves in most cameras don’t correspond exactly to the true 709 monitor specs as everyone would be up in arms if the camera only had a 5 stop range.

With almost any reasonably up to date cameras, anyone comparing a couple of cameras dynamic range with REC-709 and saying A has more dynamic range than B is a bit of a misnomer. Yes there will be differences in the way the gamma curve has been interpreted by the manufacturer or the way the knee works and perhaps camera A will give a more pleasing picture than B, but this is unlikely to actually be due to the dynamic range of the camera, just different ways the manufacturer has chosen to interpret the restrictions imposed by REC-709.

In my opinion, the only times you would want to use 709 is when you are shooting direct for broadcast where there won’t be time for grading or if you have enough control over your lighting to stay within a 6 stop range and that 6 stop range gives you the look you want.

Otherwise, I would use one of the extended range gammas included in most cameras these days, log, cinegamma, hypergamma etc, as these extended range gammas don’t try to comply to the REC-709 specifications they are not restricted to the legacy dynamic range imposed on us by REC-709. As a result they can easily accommodate much greater dynamic ranges and get closer to utilising the camera sensors full range, but at the expense of requiring some grading to restore contrast.

cameras, Uncategorized

Canon C-Log on the C300 compared to S-Log.

December 31, 2011 alisterchapman 5 Comments

First let me say that as yet I have not used C-Log in anger, only seen it at a couple of hands on demo events and in downloaded clips.

From what I’ve seen C-Log and S-Log are two quite different things. S-Log on the F3 is a true Log curve where each stop of exposure is recorded using roughly the same amount of data and the available dynamic range is about 13.5 stops. It is inevitable that when you use a true log curve like this and play it back on an uncorrected Rec-709 (standard HD gamma) monitor that it will look very flat and very washed out. This is a result of the extreme gamma miss-match across the entire recording range. If you had a monitor that could display 13.5 stops (most only manage 7) and the monitor had a built in Log curve then the pictures would look normal.

What has too be considered is that S-Log is designed to be used with 10 bit recording where each stop gets roughly 70 data bits ( this roughly means 70 shades of grey for each stop).

Now lets consider the Canon C300. It has no 10 bit out, it’s only 8 bit. Assuming Canon’s sensor can handle 13.5 stops then using 8 bit would result in only 17 bits per stop and this really is not sufficient, especially for critical areas of the image like faces and skin tones. A standard gamma, without knee, like Rec-709 will typically have a 7 stop range, this is a deliberate design decision as this yields around 34 bits per stop. As we know already if you try to do a hard grade on 8 bit material you can run in to issues with banding, posterisation and stair stepping, so reducing the bits per stop still further (for example by cramming 13.5 stops into 8 bits) is not really desirable as while it can improve dynamic range, it will introduce a whole host of other issues.

Now for some years camera sensors have been able to exceed 7 stops of dynamic range. To get around the gamma limitation of 7 stops, most good quality cameras use something called the knee. The knee takes the top 15 to 20% of the recording range to record as much as 4 to 5 stops of highlights. So in the first 0 to 80% range you have 6 stops, plus another 4 to 5 stops in the last 20%, so the overall dynamic range of the camera will be 10 to 11 stops.

How can this work and still look natural? Well our own visual system is tuned to concentrate on the mid range, faces, foliage etc and to a large degree highlights are ignored. So recording in this way, compressing the highlights mimics they way we see the world, so doesn’t actually look terribly un-natural. OK, OK, I can hear you all screaming… yes it is un-natural, it looks like video! It looks like video because the knee is either on or off, the image is either compressed very heavily or not at all, there is no middle ground. It’s also hard to grade as mid tones and highlights have different amounts of squashing which can lead to some strange results.

So the knee is a step forward. It does work quite well for many applications as it preserves those 34 bits of data for the all important mid tones and as a result the pictures look normal, yet gives a reasonable amount of over exposure performance. Next came things like cine gammas and film style gammas.

These often share a very similar gamma curve to standard gammas for the first 60-70% of the recording range, so faces, skin, flora and fauna still have plenty of data allocated to them. Above 70% the image becomes compressed, but instead of the sudden onset of compression as with a knee, the compression starts very gently and gradually increases more and more until by the time you get close to 100% the compression is very strong indeed. This tends to look a lot more natural than gamma + knee, yet can still cope with a good over exposure range, but depending on the scene it can start to look a little flat as your overall captured range is biased towards highlights, so your captured image contains more bright range than low range so will possibly (but not always) look very slightly washed out. In my opinion, if shooting with cinegammas or similar you should really be grading your material for the best results.

Anyway, back to the Canon C300. From what I can tell, C-Log is an extension of the cinegamma type of gamma curve. It appears to have more in common with cinegammas than true S-log. It looks like the compression starts at around 60% and that there is a little more gain at the bottom of the curve to lift shadows a little. This earlier start to the compression will allow for a greater dynamic range but will mean fewer bits of data for skin tones etc. The raised lower end gain means you can afford to underexpose more if you need to. As the curve is not a full log curve it will look a lot more agreeable than S-Log on an uncorrected monitor, especially as the crucial mid tone area is largely unaffected by strong compression and thus a large gamma miss-match.

For the C300 this curve makes complete sense. It looks like a good match for the cameras 8 bit recording giving a decent dynamic range improvement, largely through highlight compression (spread over more recording range than a conventional knee or cinegamma), keeping mid tones reasonably intact and a little bit of shadow lift. Keeping the mid range fairly “normal” is a wise move that will still give good grading latitude without posterisation issues on mid range natural textures.

Uncategorized

More Codec and Gamma Tests.

December 7, 2011 alisterchapman 7 Comments

More Gemini, Samurai, AC-Log and S-Log sample frame grabs. See download box at bottom of post.
I had thought, when I first wrote this post that I had discovered a strange issue where the 444 RGB recordings from the Gemini had more dynamic range than 422 recordings. I didn’t think this was right, but it was what my NLE’s (FCP and CS5.5) were telling me. Anyway to cut a long story short, what was happening was that when I dropped the Gemini RGB files into the timeline the levels got mapped to legal levels, i.e. nothing over 100% while the YCbCr 422 clips go into the timeline at their original levels. The end result was that it appeared that the 422 clips were clipping before the 444 clips. Thanks to Waho for suggesting that it may be a conversion issue with the frame grabs, I was able to see that it was simply the way the NLE’s (both CS5.5 and FCP were behaving in the same way) were clipping off anything in the 422 clips above 100% both in the frame grabs and also on the monitor output. As the RGB files were all below 100% they were not clipped so appeared to have greater dynamic range.

Anyway….. below is a new set of frame grabs layered up in a single photoshop file showing how the various codecs and recorders and codecs perform. The levels in these have been normalised at 100% to avoid any dodgy clipping issues. I’ve included F3 Cinegamma 4, plus my AC-Log picture profile, plus Samurai ProRes, Gemini S-Log and F3 Internally recorded S-Log of a very extreme contrast situation. Use the link below to download the photoshop layers file. You’ll need to me a registered user to access the link.

[downloads_box title=”More codec test grabs.”]
Photoshop Layered Frame Grabs v3
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Alister Chapman, Cinematographer, DIT, and Educator's Personal Blog

Tag Archives: gamma

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

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

What is a Gamut or Color Space and why do I need to know about it?

What’s the difference between Latitude and Dynamic Range?

Choosing the right gamma curve.

To shoot flat or not to shoot flat?

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

REC-709 – A true REC-709 camera should only have 5 stops of dynamic range!

Canon C-Log on the C300 compared to S-Log.

More Codec and Gamma Tests.

Cinematographer and film maker Alister Chapman's Personal Blog Blog