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.
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.