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GAMMA SETTING


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#1 Ale Reynoso

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Posted 25 May 2007 - 10:48 PM

Hello:
I cannot get the exact relation between the master gamma adjustment and the color saturation.
It is said if you set the gamma crossover lower than STD ( 18% grey at 45 IRE or .6V) you get richer, more saturated colors. And the opposite if you raise the m gamma. http://www.sonybiz.n...C RefInfo DVCAM
As far as I know, by modifiing the gamma setting you are modifing the mid tonal response, giving more room to the shadows or the highlights between the video signal range, leaving the white and blacks unmodified. But the color saturation is independent of the tonal response.

Am I right? What I´m missing?

Isn´t one of the main reasons for lowering the gamma to give more room to the highlights, as they loose detail easily in video?
Couldn´t be a problem not having the mid grey "middle"? (considering you are modifiing the "zone system". Using a spotmeter for exposure could give you distorted and tricky values).

Anyway, I know is a very usefull and valuable parameter. I.E. to fit the ilumination range of a scene into the video range (in conjunction with knee/black gamma/master black), besides the aesthetical values.

Just a few thinkings about gamma...

Thanks for any input
Best regards
Alejandro
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#2 Alessandro Machi

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Posted 26 May 2007 - 04:18 AM

.........But the color saturation is independent of the tonal response.

Am I right? What I´m missing?


Thanks for any input
Best regards
Alejandro


I think color saturation is completely dependent on tonal response. If I am correctly assuming that when editing a video image, set-up and video level adjustments affect the tonality of the shot which in turn is also affecting the saturation of the chroma. That's primarily why I believe in external and individual dials (in the editing/color correction stage) for these separate adjustments because it's much faster to have an individual knob that adjusts each parameter instantly and separately.

I think it's crazy to do it any other way when editing.
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#3 Bruce Greene

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Posted 26 May 2007 - 11:15 AM

As far as I know, by modifiing the gamma setting you are modifing the mid tonal response, giving more room to the shadows or the highlights between the video signal range, leaving the white and blacks unmodified. But the color saturation is independent of the tonal response.

Am I right? What I´m missing?

Thanks for any input
Best regards
Alejandro



Alejandro,

You are correct...and you are missing something ;)

For the best illustration of what happens when you apply the gamma control, open the curves adjustment in Photoshop (if you have it) on an image and 'tug' the curve from the middle, up and down, and see what happens to the image.

A few things though...
1. when you lower the gamma value in the camera menu, you will be brightening and lowering the contrast of the image for the values between 50% and 100% white. That is because 50% will get the most change and 100% will get no change.

2. when you lower the gamma value in the camera menu, you will be brightening and increasing the contrast of the image for the values between 50% and 0% white. (0%=black)

3. The exact opposite happens when you raise the gamma value in the camera menu.


Think about this: If you have a medium looking red color with RGB=100-50-50 and raise the gamma to "darken" the picture, the same pixel will have a value of perhaps RGB=100-20-20. A much more saturated red. Of course if in the very same image you have a pixel of a rather dark red of RGB=50-0-0 it will change to RGB=20-0-0, a darker and less saturated red!

So the same gamma adjustment both increases and decreases saturation at the same time!
It also increases and decreases contrast at the same time.

Hope this is clear :huh: ,
-bruce
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#4 Michael Nash

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Posted 28 May 2007 - 04:05 PM

If often raise the gamma (brighten the midtones) to smooth out the "rolloff" or transition into highlights, and then close down the iris a little to restore proper midtone exposure. If need be I'll raise the black stretch (not master black) to restore some of the darkened shadows from the lowered exposure. This lowered contrast also lowers the appearance of color saturation, so you may want to adjust color saturation as well (I usually do that by eye/monitor, but you could use a chart and a vectorscope for more accuracy)/

Also keep in mind that some of the "gamma" settings on some cameras (especially with names like "cinelike" or "filmlike") also incorporate changes to several parameters like knee and black stretch, and not just the master gamma or midtones.
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#5 Ale Reynoso

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Posted 30 May 2007 - 11:53 PM

If often raise the gamma (brighten the midtones) to smooth out the "rolloff" or transition into highlights, and then close down the iris a little to restore proper midtone exposure. If need be I'll raise the black stretch (not master black) to restore some of the darkened shadows from the lowered exposure. This lowered contrast also lowers the appearance of color saturation, so you may want to adjust color saturation as well (I usually do that by eye/monitor, but you could use a chart and a vectorscope for more accuracy)/

Also keep in mind that some of the "gamma" settings on some cameras (especially with names like "cinelike" or "filmlike") also incorporate changes to several parameters like knee and black stretch, and not just the master gamma or midtones.


I think the key word is appearance. You are not saturating or desaturanting the colors dramatically, but only in appearance. So I´ll keep handling the tonal response with gamma, and saturation with linear matrix.
Thank you!!
Best regards
Alejandro
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