While you can adjust the slope (gain), black gamma and knee of the standard gammas these are still very different to the way the cinegammas work. The key difference is that the highlight compression applied by the cinegammas starts very gently around 50% and the progressively increases as you go up the exposure range. If you try to use the knee to get a similar amount of highlight compression you have to use either a very low knee point or a steep slope or maybe even both. The problem with the knee is that it is either on or off, your highlights are either compressed or not, there is no middle ground. The more compression you add to any part of an image, the less well it tends to grade.
Where you have controlled lighting or a restricted dynamic range then standard gammas with minimal knee work very well and would be my recommendation. But where you have less control over the lighting or are dealing with a high dynamic range scene then the cinegammas tend to deal with overexposure in a much more natural way than the knee. A typical knee artefact would be an ugly highlight on a face that suddenly overexposes and looks wrong because you go from normal exposure below the knee to compressed range for the overexposure almost instantly. A cinegamma will handle the overexposure in a more progressive manner.
If your worried about loss of shadow detail with the cinegammas you can use black gamma with the cinegammas, just as you would with a standard gamma, so by using a cinegamma combined with black gamma you gain the benefits of progressive overexposure control with adjustable shadow and mid range control.
Middle grey with the Cinegammas should be around 42-44%. Exposure with cinegammas should be alittle lower than with 709 as you want to avoid putting faces etc too high up the exposure range to keep them in the more linear part of the curve. It should be remembered that if your using a monitor or TV with Rec-709 or similar then whenever you use a camera gamma that doesn't match that display gamma you will have a miss-match so you are moving away from the optimum settings for 1:1 reproduction of the scene. The further away you go from the display gamma the bigger the discrepancy. Sometimes this is desirable and helps create a "look", but sometimes as with a cinegamma or any other gamma that captures a greater range than the display gamma was designed to show it will result in a flat looking image that may then need grading. Cinegammas were designed to maximise dynamic range to offer maximum flexibility in post, they are not really optimum for "direct to air" type productions.
Alister Chapman
http://www.xdcam-user.com
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