I don't understand your question. Where exactly are you seeing this?
If you go to settings -> image control -> gamma correction you can drag that onto a video clip and you will see in the video effects window an option to adjust the gamma value for the clip. The range of values is from 1 to 28. My understanding of gamma is that it is a way to adjust for sensor responses between different cameras (or screens) due to the fact that the sensors responses are not linear, but exponential. The usual value of gamma is 2.2, but adobe default is 10. A gamma of 1.0 would mean to me that the response is lineal. Clearly Adobe is using different scale, and I would like to know how it translates to real gamma values.
Thank you for responding. I hope my question is clearer
What? Settings? Image Control? Gamma Correction?
Where are you? Not Premiere, surely!
Gamma is midtone. Pedestal, gamma and lift control the contrast of the source material.
I hope my question is clearer
Not really. I'm still lost. Where exactly are you seeing these settings?
The OP is referring to the Gamma Correction effect:
As far as the answer to the question: I have no idea. I don't see why it would matter. Color correction is not something you usually do "by the numbers," anyway. If your image looks the way you want it to look, who cares what the translation between a gamma curve and an effect's likely arbitrary number is? Just change the values until you get the result you want.
FWIW: I've never seen the need for this effect. There are other effects that let you manipulate gamma with far more finesse than this. It's rather brutish.
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Ah, I see. I never knew it was there ;-(
However, if you study the waveform monitor when you apply it, and before you've made an adjustment, you'll see that it takes the whites down to legal levels - under 100 IRE. It probably does the same (in reverse) to crushed blacks.
Much better tools for luma correction are pedestal/ lift gamma/midtones and gain/highlights control in either RGB Correction or Luma Corrector.
The 2.2 gamma you talk about is monitor "brightness", to simplify, and affects how an image appears on your local monitor(s). Gamma in video correction affects the luma component of the source material itself, no matter what your monitor gamma is set to.
Thank you for your responses.
A few of you question my motivation for wanting to know how this control works. I agree that it may be 'brutish', but I am actually trying to use this control in my research into video copy detection techniques. My system needs to be robust to many transformation such as resizing, color changes, contrast changes, and others including changes in gamma. I do not want fine control. I just want to know what the scale means.
If I start whith a gamma of 10 and do a gamma correction to 5, have I halved the Luma component? Similarly, if I go from 10 to 20 what does that indicate.
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No! It all depends upon the contrast range of the source material.
Use the waveform monitor and look at your matreial as you adjust the gamma. That is what it's for.