Understanding Sony’s Viewfinder Display Gamma assist.

Most of sony’s cameras that support S-Log3 or Hybrid Log Gamma also have a function called Viewfinder Display Gamma Assist.

Viewfinder Display Gamma Assist allows you to monitor with the cameras built in LCD screen or viewfinder with the correct brightness and contrast range when using gamma curves that are not directly compatible with these Rec-709 screens.

Whenever you try to view a gamma curve that is not normal Rec-709 on a Rec-709 screen the brightness and contrast that you will see will be incorrect. The most common scenario is perhaps viewing S-Log3 without any form of LUT. In this case the images will look less bright and have less contrast than they should and this makes judging exposure difficult as well making it less easy to see focus errors.

With a camera like the FX6 or FX9 most people will use the cameras CineEI mode and add a LUT to the viewfinder image to convert the S-Log3 to something that looks more contrasty and on the FX6 and FX9 the default LUT is “s709”.  However s709 is not the same thing as Rec-709 (Note that with the FX6, because LUTs are always available in the CineEI mode, viewfinder display gamma assist is not available in the CineEI mode of the FX6, you should instead use a LUT).

I think a lot of people think that the default s709 LUT is the same as Rec-709, it’s not, it is very different. They look very different and result in quite different brightness levels when exposed correctly. s709 when exposed correctly will put skin tones somewhere around 50-60% and white at 78%. If you expose s709 using normal Rec-709 brightness levels (70% skintones, 90% white) this is actually over exposed by just over 1 stop. As a result if you expose the s709 LUT, using Rec-709 levels, and then turn off the LUT and instead use Viewfinder Gamma Assist, the gamma assist will look wrong, it will be too bright and may look washed out and this is simply because the exposure IS wrong.

Almost always, if the viewfinder display gamma assist looks wrong, the exposure is wrong. When it looks right, the likelihood is the exposure is right.

A few things to understand:
  • The viewfinder is a Rec-709 range display device only capable of showing Rec-709 range and colour.
  • Feed true Rec-709 to a Rec-709 device and you will have a correct looking image with “normal” brightness, contrast and colour.
  • Feed S-Log3 to a Rec-709 device and you will have an incorrect dull, flat looking image due to the gamma miss-match between the capture gamma and display gamma.
  • Feed S-Log3 to a device with S-Log3 gamma and you will once again have the correct brightness and contrast as there is no longer a gamma miss-match (S-Log3 only appears to be flat due to the gamma missmatch between S-Log3 and Rec-709, use the right gamma and you will see that it is not actually flat).

Viewfinder Display Gamma Assist works by changing the gamma curve used in the Viewfinder to a gamma curve similar to S-Log3. When you view S-Log3 with a monitor with S-Log3 gamma you will have the correct contrast and brightness, so correct exposure will look correct.

But because the cameras LCD display screen can only show 6 to 7 stops you don’t get the full S-Log3 viewing range, just the central mid range part that is the direct equivalent of Rec-709. This very closely matches what you see if you use the Sony 709(800) LUT to convert the S-log3 to 709. The 709(800) LUT converts S-Log2 or S-Log3 to vanilla Rec-709 (70% skintones/90% white) with a knee that provides a slightly extended highlight range. It is broadly comparable to how most conventional Rec-709 cameras will look. So as a result viewfinder display gamma assist and Sony’s 709(800) LUT’s will look almost identical, while the s709 LUT will (and should by design) look different.

Viewfinder Display Gamma Assist is extremely useful for scenarios where you do not have a LUT option such as when shooting in CineEI in HD with the FX9. It can help you make good exposure assessments. It can make it easier to see when you are in focus. But it isn’t a LUT, so can’t be applied to the cameras outputs, only the built in viewfinder. Additionally if you use zebras, the waveform or histogram, gamma assist has no effect on these so you must remember that you are still measuring the levels of the actual recording gamma, not Rec-709 levels.

Viewfinder Gamma Assist is useful not only for shooting with S-Log but also when shooting using HLG (Hybrid Log Gamma). HLG is an HDR gamma curve and because the LCD viewfinder isn’t HDR you can’t correctly monitor HLG directly. Viewfinder Gamma Assist allows you to monitor with the correct brightness and contrast when shooting HLG making it easier to confidently get the correct exposure levels, as much like S-log3 the levels required for the correct exposure of HLG are quite different to Rec-709.

One last thing: NEVER use Viewfinder Gamma Assist with a LUT at the same time, this will result in a completely incorrect looking image and could result in very bad exposure as a result.

19 thoughts on “Understanding Sony’s Viewfinder Display Gamma assist.”

  1. Just a doubt… I don’t know if I am doing something wrong… But in CINE EI mode with the lut off, when I press the Gamma Display Assist (GDA) button I receive the message CANNOT PROCEED. Looks like the GDA only works when the target display, in custom mode, is set to HLG. Can you please help me?

    1. I am guessing this is an FX6. GA only works with HDR mode on the FX6 as LUT’s are always available no matter what in the CineEI mode.

  2. Totally agree with this–> “Viewfinder Display Gamma Assist is extremely useful for scenarios where you do not have a LUT option”

    I would add it’s the ONLY option when shooting FX9 in 1080P as s709 LUT is not available….which is the only design/engineering issue I’ve ever had with FX9. Any idea of potential of firmware update to allow LUTs when shooting 1080P?

    While I do find the Gamma Assist very helpful, I’d love to be able to apply LUTs to viewfinder and SDIs when shooting 1080P. Otherwise, Gamma Assist is only option available and it’s quite good.

  3. Sorry if this double comment entry…had an issue posting….

    But I would say that while I totally agree with this statement: “Viewfinder Display Gamma Assist is extremely useful for scenarios where you do not have a LUT option. It can help you make good exposure assessments.”

    I also need to mention that it’s the only option when shooting with the FX9 in 1080P. In 1080P the FX9 doesn’t allow for the use of LUTs which means the only option when using viewfinder is the Gamma Assist which is quite good but I would much rather prefer the perfect setup the camera allows when shooting 4k in CineEI which is to shoot slog3 and use the awesome s709 LUT on viewfinder and SDIs. That’s the perfect solution which frustratingly doesn’t exist when shooting 1080P. I don’t understand the design/engineering decision to do this as this is the only major issue I’ve ever had with FX9. Just have projects require 1080P and it messes with with my workflow not being able to use slog3 w/ s709 which is such an elegant and perfect solution. Any idea or thoughts if this is something Sony could fix with a firmware upgrade?

    Thanks for you time

    1. The FX9’s lack of independent LUT’s when shooting HD is well known and an unfortunate hardware limitation of the FX9 that almost certainly can’t be fixed via firmware. I would appear that the processor that normally provides the independent LUT’s in 4K is the one that’s used to provide the 4K to HD downscale for the internal recordings. I would guess that Sony assumed most people shooting log would also want UHD/4K as this will always deliver the highest quality.

      1. Yeah it’s a fair assumption Sony made although I think they underestimated the amount of tv & documentaries that still shoot in 1080P. But so it goes….thanks for your response and time.

    2. I have the same issue in 1080P. It makes zero sense, especially for operators wanting to use their own monitors. I mostly get SmallHDs for video village and their calibrated Slog to Rec 709 LUTs look like complete trash. Has anyone found a good solution to this? Any custom LUTs I could possibly load onto the Small HDs that have accurate colors?

      1. You can use LutCALC to convert normal LUT’s to LUTs suitable for the Small HD monitors. Small HD require both full range in and very unusually – full range out.

  4. Hi,

    Because GA is not available in Cine EI mode on the FX6; when using CineEI mode and shooting S-Log3, would is be best therefore to add a Rec709 custom LUT to the camera and turn off the viewfinder/monitor LUT?

    Would you know if GA has an effecter on the FX6 waveform display as MLUT (on) for the s709 LUT?

    Thank you,

    1. Why do you not want to use the LUT for the VF? Without the LUT CineEI does nothing.

      VF GA has no effect on waveform or anything else as it is purely an adjustment to the viewfinders gamma curve.

      1. Because the MLUT is s709 and not Rec709. Wasn’t that the point you were making? Rec709 being better than s709? Or perhaps I misunderstood?

        Does GA apply rec709 to the displayed image?

        Then again, if the waveform is unaffected by GA I’m not keen.

  5. Thanks a lot for clearing this up in greater detail. Much appreciated.

    I actually fully rely on VF GA by now, so that the waveform measures SLog3 and I do not send a LUT to my external SmallHD. I can then apply a Rec709 LUT in the monitor itself, or record SLog3 without a baked in LUT with an Atomos recorder.

    Works like a charm and just as I want it.

    I believe it’s the only way to do it if you want to send SLog3 without a LUT applied out via SDI…

  6. Hello Alistair and everyone,

    About Hybrid Log Gamma: this is the curve I use most often with the PXW-FS5 (Rec-709, HLG3), as it allows me to easily calibrate my images according to the desired rendering, with simple adjustments (exposure, contrast, saturation…). Most of the time, I don’t use a predefined LUT. However, despite my tests, I still have a bit of trouble determining whether it is better to overexpose by one or two stops at the shoot, as it is recommended to do in S-Log2 or S-Log3, because I usually manage to “recover” a correctly exposed image in post-production, with or without overexposure at the shoot. It must depend on the shooting conditions ? Any opinions from users of the HLG3 curve ?

    1. HLG is nothing like a log gamma curve such as S-Log3. It is much more like Rec-709 than anything else, so it grades more like 709 and you can’t expose it brighter without pushing the highlights and brighter parts of the image into the compressed highlight range which then makes it very difficult to get a pleasing result.

      1. Thank you very much Alister for your feedback.
        Yes, it is true that despite my hesitations with these settings, I had noticed that overexposure was not the strong point of the HLG curves. Their appreciable dynamic range, on the other hand, allows a little leeway in post-production.

        But I take good note to avoid playing the overexposure card with these HLG profiles…

        1. Dynamic range isn’t just highlights. What everyone forgets is that the ONLY way you can make more space for a bigger highlight is by bringing mid range down to make space. There is nothing that can be done to avoid this and this is why the Hypergammas must be exposed darker if you want to gain a greater highlight range. If you expose them equally brightly as any other gamma then you will have exactly the same space for highlights as any other gamma, so the highlight range remains very similar.

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