Tag Archives: EI

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

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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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Camera Gain: It doesn’t make the camera more sensitive! (also relevant EI S-Log).

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This is something that’s not well understood by many people. It helps explain why the PMW-F3 (and other cameras) EI S-Log function is so useful.

You see, camera gain does not normally actually change the cameras ability to capture photons of light. A CCD or CMOS sensor has a number of photo sites that capture photons of light and convert those photons into electrons or electrical charge. The efficiency of that capture and conversion process is fixed, it’s known as the QE or quantum efficiency. There are a  lot of factors that effect this efficiency, such as the use of micro lenses, whether the sensor is back or front illuminated etc. But all of these factors are physical design factors that do not change when you add extra camera gain. The sensitivity of the sensor itself remains constant, no matter what the camera gain is set to.

Camera gain is applied to the signal coming out of the sensor. It’s a bit like turning up the volume on a stereo amplifier. If you have a quite piece of music, turning up the volume makes it louder, but the original piece of music is still a quiet piece of music. Turning up the volume on your stereo, as well as making the music louder will also make any hiss or background noise in the music louder and it’s exactly the same with a video camera. As you increase the gain, as well as the wanted video signal getting bigger (brighter) all the unwanted noise also get bigger. So adding gain on your video camera doesn’t actually make the camera more sensitive, but it does make what light the camera has captured brighter in the recordings and output, giving the impression that the camera has become more sensitive, however this is at the penalty of increased background noise.

As well as adding gain to the image in the camera, we can also add gain in post production. Traditionally gain has been added in camera because the gain is added before the recording process. In the uncompressed analog days the recording process itself added a lot of noise. In the digital age the process of compressing the image adds noise.  8 bit recordings have quite small number of grey shades. So any gain added in post production amplifies not only the camera signal but also the added recording or compression noise so generally gives an inferior result to adding gain in camera. With an 8 bit signal the stretching of the relatively few grey shades results in banding.

Now, however the use of lower noise sensors and much improved 10 bit or higher recording codecs or even uncompressed recording means that adding gain in post as opposed to in camera is not such a bad thing. In some cases you can use post production noise reduction prior to adding post gain and by leveraging the processing and rendering power of a computer, which will normally be of greater quality than the in camera processing, you can get a cleaner, lower noise output than you would using in camera gain. So before you flick on the gain switch of your camera, if your using only very light 10 bit or higher compression (HDCAM SR, Cineform, ProRes HQ) or uncompressed do consider that you may actually be better waiting until you get into post before you add gain.

Some modern cameras, like Red or the Sony F3 can use something called EI gain. EI gain does not actually add any gain to the recorded signal (or signal output in the case of the F3). Instead it adds gain to the monitor output only and adds metadata to the recording to tell the post facility or conversion software to add gain. This way you see on the monitor what the image should look like when the gain has been added, but the recording itself has no gain added giving the post production team the ability to fine tune exactly how much gain is applied.

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EI S-Log. What’s different?

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You may have seen that Sony are releasing a free firmware update in the next week or so for the PMW-F3 that incorporates a new version of S-Log. You will still need to have purchased the S-Log upgrade in order to use S-Log, but now there are two variations of S-Log, normal S-Log and EI S-Log.

EI S-log differs from the original S-Log in that the base ISO is 800 as opposed to the 1600 of the original. You can select either S-Log or EI S-Log mode in the menu. When EI S-Log is selected you have the ability to then add EI gain to the MLUT’s (Monitor Look Up Tables or LUT’s). When you switch the EI ISO to 1200 for example, the additional gain is added to the LUT to give the equivalent S-Log + 3db gain output on the monitor out and to the SxS card recordings as using S-Log with gain. But the actual S-Log output on the A-B dual link outputs remains fixed at 800 ISO. The benefit of this is that what you see on the monitor out represents what you will end up with after post production with added gain or lift, it’s a way of pre-visualising what you will finish up with, without compromising your recordings.

Theoretically, if the cameras native 0db gain point is represented by 800 ISO. Which is what you have when using 50/60i standard gamma at 0db. Reducing the cameras S-Log gain like this by 6db compared to the previous or standard S-log base ISO of 1600. Should yield a 6db (1 stop) dynamic range improvement. Given that S-Log already improves the dynamic range by about 1.5 stops, then on paper at least, EI S-Log should yield a  2.5 stop improvement over the 11 stops the standard and cinegammas give. That would make the F3 a camera capable of 13.5 stops which is quite remarkable. I hope to be able to measure the actual DR very soon and see if this is really the case. Anyway, whatever the outcome of the DR measurements, the EI option is a nice one to have as it will allow you to underexpose a little (to gain extra headroom) when you shoot and then use the added EI LUT gain to check that even after gain gets added in post the images will still be noise free enough for use in your production.

A further feature of the update is the ability to change the R and B gain when shooting in S-Log. This will allow you to tweak your white balance. Currently when shooting s-Log you can only use a preset white balance dialled in in 100k steps, there is no option to white balance the camera using a grey/white card.

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You may have seen that Sony are releasing a free firmware update in the next week or so for the PMW-F3 that incorporates a new version of S-Log. You will still need to have purchased the S-Log upgrade in order to use S-Log, but now there are two variations of S-Log, normal S-Log and EI S-Log.

EI S-log differs from the original S-Log in that you can select either S-Log or EI S-Log mode in the menu. When EI S-Log is selected you have the ability to then add EI gain to the MLUT’s (Monitor Look Up Tables or LUT’s). When you switch the EI ISO to 1200 for example, the additional gain is added to the LUT to give the equivalent S-Log + 3db gain output on the monitor out and to the SxS card recordings as using S-Log with gain. But the actual S-Log output on the A-B dual link outputs remains fixed at 800 ISO. The benefit of this is that what you see on the monitor out represents what you will end up with after post production with added gain or lift, it’s a way of pre-visualising what you will finish up with, without compromising your recordings.

Theoretically, if the cameras native 0db gain point is represented by 800 ISO. Which is what you have when using 50/60i standard gamma at 0db. Reducing the cameras S-Log gain like this by 6db compared to the previous or standard S-log base ISO of 1600. Should yield a 6db (1 stop) dynamic range improvement. Given that S-Log already improves the dynamic range by about 1.5 stops, then on paper at least, EI S-Log should yield a  2.5 stop improvement over the 11 stops the standard and cinegammas give. That would make the F3 a camera capable of 13.5 stops which is quite remarkable. I hope to be able to measure the actual DR very soon and see if this is really the case. Anyway, whatever the outcome of the DR measurements, the EI option is a nice one to have as it will allow you to underexpose a little (to gain extra headroom) when you shoot and then use the added EI LUT gain to check that even after gain gets added in post the images will still be noise free enough for use in your production.

A further feature of the update is the ability to change the R and B gain when shooting in S-Log. This will allow you to tweak your white balance. Currently when shooting s-Log you can only use a preset white balance dialled in in 100k steps, there is no option to white balance the camera using a grey/white card.