Correct Exposure with Cinegammas and Hypergammas.

When an engineer designs a gamma curve for a camera he/she will be looking to achieve certain things. With Sony’s Hypergammas and Cinegammas one of the key aims is to capture a greater dynamic range than is possible with normal gamma curves as well as providing a pleasing highlight roll off that looks less electronic and more natural or film like. To achieve these things though, sometimes compromises have to be made. The problem being that our recording “bucket” where we store our picture information is the same size whether we are using a standard gamma or a more advanced gamma curve. If you want to squeeze more range into that same sized bucket then you need to use some form of compression. Compression almost always requires that you throw away some of your picture information and Hypergamma’s and Cinegamma’a are no different. To get be able to record this greater range, the highlights are compressed.

exposure2-300x195 Correct Exposure with Cinegammas and Hypergammas.
Compression point with Hypergamma/Cinegamma.

To get a greater dynamic range than normally provided by standard gammas the compression has to be more aggressive and start earlier.

With Rec-709 we normally record white (a white card or white piece of paper) at between 85 and 90%. That only leaves the space between 90% and 109% (or 100% if the clip point has been set to 100%) to record anything that is brighter than the white piece of paper. That really isn’t much space for a bright sky, shiny cars or lots of highlights. So with the Hypergammas and Cinegammas we bring the white recording level down to between 70 and 75%. This them means you have much more space between 75% and 109% to record a larger range of highlights in a much more pleasing manner with a more gentle roll off.

But, this lower white level  means you really need to watch your exposure. It’s ironic, but although you have a greater dynamic range i.e. the range between the darkest shadows and the brightest highlights that the camera can record is greater, your exposure latitude is actually smaller, getting your exposure just right with hypergamma’s and cinegamma’s is very important, especially with faces and skin tones.

The correct exposure for skin tones with the Hypergammas and Cinegammas is around 55-60%. So instead of setting zebras to 70% for skin tones, you should set them to 55% to 60%. If you overexpose a face (by exposing it at 70% or more)  then you start to place those all important skin tones in the compressed part of the gamma curve, at the start of the roll off. It might not be obvious in your footage, it might look OK. But it won’t look as good as it should. Skintones will start to loose contrast and look a bit flat if above 70%. This can then make it might be hard to grade. It’s often not until you compare a correctly exposed shot with a slightly over shot that you see how the skin tones are becoming flattened out by the gamma compression.

But what exactly is the correct exposure level? Well I have always exposed Hypergammas and Cinegammas so they look 10 to 15% darker than a 709 image would. This isn’t “under exposed” this is actually the correct exposure. If you set the camera to Rec-709 and then change the gamma curve to a Hypergamma you will see the image get a touch darker, even though the ISO or gain may not change.  So if faces are sitting around 70% with a standard gamma, then with HG/CG I expose that same face at 58%. In both cases you should find the aperture will be the same – the exposure is the same, even though the brightness is slightly different. This has worked well for me although sometimes the footage might need a slight brightness or contrast tweak in post the get the very best results.

On the Sony FS7, F5 and F55 cameras Sony present some extra information about the gamma curves.

Hypergamma 3 is described as HG3 3259G40 and Hypergamma 4 is HG4 4609G33.

What do these numbers mean? lets look at HG3 3259G40
The first 3 digits – 325 is the dynamic range in percent compared to a standard gamma curve, so in this case we have 325% more dynamic range, roughly 2.5 stops more dynamic range. The 4th number which is either a 0 or a 9 is the maximum recording level, 0 being 100% and 9 being 109% (If using a 109% hypergamma also check that the white clip is set to 109%, sometimes it’s set to 105% so the very top part of the curve gets clipped).

100% may be necessary for some analog broadcasters, so for direct to air productions you should stick to HG1, HG2 or CG2.

Finally the last bit, G40 is where middle grey is supposed to sit. With a standard gamma, if you point the camera at a grey card and expose correctly middle grey will be around 43%. So as you can see these Hypergammas are designed to be exposed a little darker. Why? Simple, to keep skin tones away from the compressed part of the curve.

Here are the numbers for the 4 primary Sony Hypergammas:

HG1 3250G36, HG2 4600G30, HG3 3259G40, HG4 4609G33.

Cinegamma 1 is the same as Hypergamma 4 and Cinegamma 2 is the same as Hypergamma 2.

15 thoughts on “Correct Exposure with Cinegammas and Hypergammas.”

  1. Thank’s, very helpful. About to order a PXW-FS7 and guess your your advice of how to expose are the same for this camera and slog3?

  2. Thanks for very useful post. I got FS7 and didn’t know about how HG works. And what these number means. So this post helped me a lot.
    With HG,I think I going to use Zebra for expose face 60%. I will set zebra 1 60%. But could you give me advice that zebra aperture must be how many present? 5% is too wide? Maybe 3 percent or something?

    And I have one more question. HG has G40 and G33 as you mentioned. Could I expose 60% to face with both G40 and G33? In my understand, I have to change percentage between G40 and G33. 60% is for G40? And how many percent with G33?

    I’m sorry for my English.

    1. Different curves require slightly different exposure levels. G40 puts skin tones around 60-65% and G33 at around 55-60%. A 5% window is fine, zebras are a guide only, skin tones vary hugely so no two faces will ever have exactly the same exposure.

  3. I shot with my SONY FS7 in HG4 – 4609G33 day exterior for the first time. I tried to expose the shot with a gray card at 33 IRE and the image was way under exposed. Flesh tones down at 45 IRE. I just went with my gut reaction and exposed them at 55 to 60 IRE. The Gray card was over exposed based on the 33 IRE. The exposure looked good but the whole image was washed out and de-satured. Almost like I was shooting SLog. What’s going on? I thought it would be closer to the final image.

    1. Exposure is NOT brightness. Exposure is letting the right amount of light fall on the sensor. Hypergammas are different to the gamma curve used in most monitors, so the image will look a bit flat and washed out until you grade the footage.

  4. Is it fair to say that a difference in IRE of 10% is equivalent to one F stop? From what you’re saying here, to get a good exposure for light skin tones, it sounds like you just get a normal exposure, then back off a stop or less.

    1. 1 stop is about 10% in the mid range. More in the upper range, less in the lower range. Depends on what you mean by “normal” exposure. Skin tone should be around 60% with Hypergammas.

  5. Hi Alister, the article says “If you overexpose a face when using these advanced gammas (and S-log and S-log2 are the same) then you start to place those all important skin tones in the compressed part of the gamma curve”

    I understand that’s the case for Cinegammas, but isn’t this different for S-Log2? In the graphs that show the S-Log2 & S-Log3 curves and how the store data relative to middle grey, the Log curves are pretty much a straight line at the bright part of the curve. So they aren’t getting more compressed, only brighter.

    1. This is an old article and the problem was that at the time of writing we didn’t have the wonderful post production tools we have today for managing log. So what used to happen was if you over exposed the log too much, the log to display gamma conversion in post would treat the extra bright skin tones as a highlight and compress them during the conversion resulting in flat and washed out highlights. Now we have tools like LUTCalc that can create LUTs that incorporate over exposure offsets or color managed workflows that have a linear intermediate stage that can happily deal with this. You also have to consider that as you go up in brightness by 1 stop, the real world brightness range doubles, while the log remains constant. So relative to the scene if you push the exposure up too high you will have less data per stop, but with exposure compensated LUT’s or a color managed workflow this is rarely a problem these days. Even now, if you use an off-the-shelf LUT or workflow that doesn’t allow you to normalise the exposure before grading, even today, exposing too bright can result in disappointing, flat looking skin tones.

  6. Thanks for the update info. Could you also please explain if there are any differences with cg3 and cg4 and middle grey exposure recommendations? I imagine cg4 has a slightly less compressed curve in the shadow region – is that right? Would that push middle grey up a little?

  7. Hi Alister,

    thanks so much for all your tutorials. I learned a lot about my new FS7 II from you!

    Maybe I could ask you to kindly clarify one thing I’m still not quite sure about:

    I’m shooting in Slog3, using the REC709(800) LUT and and EI of 800.
    According to your recommendations, I kept my skin zebra at 70%, which seems to work well. Now, I’m wondering wether I should set my 2nd zebra to 90% or 100% to prevent white clippings in the sky etc. Assuming that Rec709(800) converts the Slog3 proportionally, a 100% zebra should be correct. But also taking into account that I’m already over-exposing by 1,5 stops with the EI800 setting: Where are my 90% whites now? When do my whites start to clip in the Slog3? And where would you set the second zebra accordingly?

    Thanks so much for you work and time. I really do appreciate!

    Kind regards,

    1. Using 100% zebras as an indication of clipping is OK, but the camera will also flag up a “highlight” warning if too much of the shot is clipped anyway. I much prefer to use the Hi/Low Key function to take a look at highlights.

      90% whites are not highlights. 90% white would be a piece of paper and if the skin tones are exposed correctly then white will also be exposed correctly as white is always approx 1.5 stops above skin tones. Clouds and sky will always be much brighter than a piece of paper, much brighter than 90%.

      Exposure should never be based on highlights. The mid range of the shot is the bit that needs to be right. The viewer will never notice a few clipped specular highlights but will sure as hell notice if the mid range is too dark or too bright.

  8. Excuse my bad english.
    The biggest problem with low end camcorders wich have those extended dynamic gamma curves is that they work with 8 bit (especially in 4k), wich is not enough and they shoot thus very visible banding artifacts. The only solution is to downgrade the clips in the standard Rec. 709 gamma curve. The issue is even more serious when they shoot in HDR.

    1. This isn’t always true. You can shoot 8 bit at higher dynamic ranges than 709 and get perfectly acceptable results. It’s all down to your workflow and care taken with exposure. Of course 10 bit is preferable, but it is possible to get good results, even with HDR starting with 8 bits.

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