Will a bigger recording Gamut give me more picture information?

advertise-here-275 Will a bigger recording Gamut give me more picture information?

The short answer is it all depends on the camera you are using. With the F55 or F65 then S-Log2/S-Gamut and S-Log3/S-Gamut3 will give you a larger range of colours in your final image than S-Log3/S-Gamut3.cine. But if you have a PMW-F5, PXW-FS7 or PXW-FS5 this is not going to be the case.

What is Gamut?

The word Gamut means the complete range or scale of something. So when we talk about Gamut in a video camera we are talking about dynamic range and color range (colorspace) taken together. Then within the Gamut we can break that down into the dynamic range or brightness range which is determined by the gamma curve and the color range which is determined by the colorspace.

Looking at the current Sony digital cinema cameras you have a choice of 3 different gamuts when the camera is in log mode plus a number of conventional gamuts you get when shooting rec-709, rec-2020 or any other combination of rec-709 color with cinegammas or hypergammas.

Log gamma and gamuts.

But it’s in the log mode where there is much confusion. When shooting with log with the current cameras you have 3 recommended combinations.

S-Gamut (S-Gamut colorspace + S-log2 gamma).

S-Gamut3 (S-Gamut3 colorspace + S-Log3 gamma).

S-Gamut3.cine (S-Gamut3.cine colorpace + S-Log3 gamma).

The S-log2 and S-log3 gamma curves both capture the same dynamic range – 14 stops, there is no difference in the dynamic range captured.

In terms of the range of colors that can be recorded S-Gamut and S-Gamut3 are the same size and the largest recording colorspaces the cameras have. S-Gamut3.cine is a smaller colourspace but still larger than P3 (digital cinema projection) or rec-709.

Gamuts-only Will a bigger recording Gamut give me more picture information?

But those were all designed for the F55 and F65 cameras that have extremely high quality (expensive) colour filters on their sensors. The reality is that the F5/FS7/FS5 sensor cannot see the full range of any of the S-Gamut colorspaces so in reality you gain very little by using the larger versions. Don’t expect to see a noticeably greater range of colours than any of the other colour modes if you have the F5/FS7/FS5. But all the LUT’s designed for these cameras are based on the S-Gamuts and if you want to mix an FS5 with an F55 in one production it helps to use the same settings so that grading will be easier. It is worth noting at this point that most natural colors do fall within Rec-709, so while it is always nicer to have a bigger color range it isn’t the end of the world for most of what we shoot.

S-Log3 is a great example of what it means to have a bigger recording range than the camera can “see”. S-log3 is based on the Cineon film transfer log gamma curve developed back in the late 1980’s. Cineon was carefully tailored to match film response and designed around 10 bit data (as that was state of the art back then). It allows for around 16 stops of dynamic range. Much later, Arri and many others then adapted Cineon for use in video cameras – The “C” in Arri’s LogC stands for Cineon.

When Sony started doing wide dynamic range cameras they developed their own log gammas starting with S-Log, then S-Log2. These curves are matched very precisely to the way a video sensor captures a scene rather than film. In addition they are matched to the sensors actual capture range, S-Log can record 13 stops as that’s what the sensors in the cameras with S-Log can see. Then S-Log2 is 14 stops as the second generation cameras can all see 14 stops. As a result of being purpose designed for a video sensor, when using S-Log2 you maximise the entire recording range because the sensor is matched to the log which is matched to the record range.

But, these curves drew much criticism from early adopters and colorists because they were very different from the Cineon curve and all the other log curves based on this old school film curve. Colorists didn’t like it because none of their old Cineon LUT’s would work as expected and it was “different”.

S-log-levels Will a bigger recording Gamut give me more picture information?
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values. Note how little data there is for each of the darker stops, the best data is above middle grey. Note that current sensors only go to +6 stops over middle grey so S-Log2 and S-Log3 record to different peak levels.

In response to this Sony then developed S-Log3 and surprise, surprise – S-log3 is based on Cineon. So S-log3 is based on a 16 stop film transfer curve, but the current cameras can only see 14 stops. What this means is that the top 14% of the gamma curve is never used (that’s where stops 15 and 16 would reside) and as a result s-Log3 tops out at 92% and never gets to the 107% that S-Log2 can reach. If Sony were to release a 16 stop camera then S-Log3 could still be used and then it would reach 107%.

Coming back to colorspace. If you understand that the sensor in the F5/FS7/FS5 cannot see the full colour range that S-Gamut or S-Gamut3 are capable of recording then you will appreciate that like S-log3 (that is larger than the camera can see and therefore part empty) many of the possible code values available in S-Gamut are left empty. This is a waste of data. So from a colourspace point of view the best match when shooting log for these cameras is the slightly smaller colorspace S-Gamut3.cine. But S-Gamut3.cine is meant to be matched with S-Log3 which as we have seen wastes data anyway. If the camera is shooting using a 10 bit codec such as XAVC-I or XAVC-L in HD there are plenty of code values to play with, so a small loss of data has little impact on the final image. But if you are recording with only 8 bit data, for example XAVC-L in UHD then this does become much more of a problem and this is when you will find that S-Gamut with S-Log2 is going to give a better result as S-Log2 was designed for use with a video sensor from day 1 and it maximises the use of what little data you have.

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15 thoughts on “Will a bigger recording Gamut give me more picture information?”

  1. Amazingly clear, concise and (relatively) comprehensive explanation Alistair. Thank you very much for taking the time to share.

    It’s interesting to me as I’ve always used Slog3 with my FS7, and have been happy with the results, but I am now interested to start exploring Slog2 with S-Gamut, to see if using more of the luminance values available gives any tangible benefit in practice.

    Cheers!

    jason

  2. Thanks Alister

    I have Sony FS5 but for my is very difficult to correct expose +1 or+2 for Slog,histogram is not too god for this.
    Haw to expose correct with FS5 to use Slog ? I film wedding,i tray to find a good PP on my FS5,i don’t have time to greading a video 3 hour.
    I read all your post & i tray to find the best solution for my..thanks. I want to learn haw to use Slog..but I did not succeed..is too difficult

    1. S-Log will always need to be graded, that’s the way it’s designed to work. Yes, shooting with S-Log can be difficult, that is the nature of the beast when the camera dose not have built in LUT’s. Adding a LUT in post could be a quick way to grade, but exposure needs to be very carefully controlled if you want to do this. My suggestion is to use a white card and zebras at 70% on the white card. But that’s difficult to do when you are run and gun shooting.

  3. Hi Alister, given three wide panchromatic sensors would a prism beam splitter camera have a better gamut than a bayer based one? Do you see any chance of HLG coming to an evolution of the PXW160/180 or PMW200. Bob

    1. Potentially yes a 3 chip camera could have a very wide gamut, but it would depend on the quality of the dichroic filters used in the prism and the IR cutt-off filter, so it isn’t guaranteed. Prisms add all kids of new image quality issues such as increased flare and chromatic abberation. The F55 camera has one of the widest color gamuts of any camera produced.

      1. HLG is only of real benefit if the sensor has at least 12 stops of DR. The 160/180 don’t have the DR. The X200 might, but only just, but whether the internal processing can do anything useful with it is another question.

  4. Hello Allistar,
    Good explanation as always.
    Could you point out in a comment or separate blog post the role of setting the right white balance and white balance tuning in relation to color and grading. maybe it has effect on wide gamut as well in relation to color.

    there are a multitude of options and settings beside just setting a K value: what to do with A-B and G-M values in relation to grading / post. Also K temp might vary over a scene from bright to shadow of from inside to outside, etc. etc.
    stefan

    1. White balance is a gain adjustment of the R and B channels. It can be done but is risky when shooting log as it can result in a reduction in dynamic range in R or B relative to G. It’s better to finish the WB in post.

  5. Hi Alistar,
    I have one more pressing point to ask you:

    My Sony Alpha A7ii (and other cameras) records in 8 bit color etc, but on checking the file specs (with a program called mediainfo) it states color primaries BT 709 and: Color range: LIMITED.
    This means only 16-235 colors are recorded. What does this mean for grading. As I understand this would implicate losing low light and highlight detail? all blacks under 16 are mashed together and all highlight over 235 is white.
    how should I see this – did not expect this from my Alpha…

    1. No, you are not loosing anything, you are just recording everything using the normal acceptable range for video.

  6. I’ve learned so much and understand what i’ve read about gammas and gamuts, but getting overwhelmed with information.
    I’m starting out with the A7sii and know that i should use SLog2 when I need 14 stops of DR, but what colorspace is best for SLog2 on the A7sii and Cine2? Am i wasting data using Slog2 with Sgamut3.cine? Ive been using Sgamut3.cine with Cine 2 as well.

    1. There’s a difference between what you CAN do and what you SHOULD do. The vast majority of LUT’s and workflows are based around having the correct combination of Gamma and Gamut. If you start mixing odd gammas and gamuts, for example SGamut3 with S-log2 you may have problems grading the footage. A lot will depend on your precise workflow. I’m not sure why you would want to use a huge recording gamut like SGamut3.cine with a cinegamma, you won’t be capturing anything extra and you are just making the workflow harder than it needs to be as you will need to bring the SGamut down to a Rec-709 color space to get the best results.

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