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[Definition] Gamma

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Old 01-31-2002, 05:31 PM
Doug Nelson's Avatar
Doug Nelson Doug Nelson is offline
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[Definition] Gamma

Gamma is a display of the contrast of an image represented by a graph. Traditionally, the vertical axis represents the output values, with the lowest point being the shadows. The horizontal axis represents the input values, with the leftmost point representing the shadows.

In a perfect theorhetical image, every input value would map to exactly the same output value. If you drew a line from each input and output value and marked where they intersected, then connected all these intersection marks, you'd have a perfectly straight line at a 45-degree angle. The gamma (or "slope") of this image would then be "1", since every input value devided by the matching output value would be 1 (any number divided by that same number is 1).

If you were to change the input or output values in such a way that the slope of the gamma curve (actually a line in this theorhetical example) were to be more than 45 degrees you'd be increasing the contrast uniformly across the entire brightness range of the image. Likewise, if you were to change the values so the slope was less than 45 degrees you'd be decreasing the overall contrast.

In the real world, most values are in the mid-tone areas, so we seek to give them good contrast for differentiation. Highlight and shadow areas are flattened to compensate, since we're less sensitive to those areas.

A graphic representation of an image with a higher midtone contrast and lower shadow and highlight contrast would resemble the letter "S", so the most common gamma curve is known as an "S curve".

When you use your image editing program to adjust "curves", it is the gamma curve you are adjusting. In other words, you're adjusting (actually remapping) the output values against the input values.

Last edited by Doug Nelson; 02-02-2004 at 09:58 AM.
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Old 01-31-2002, 06:56 PM
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Thanks, I've always wondered exactly what "gamma" is. But isn't gamma also used as a term for the middle slider in the levels dialogue box (PS)? Does moving that slider create the equivalent of an S-curve? If not, what does it do - and is it really a "gamma" adjustment?

Jeanie
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Old 01-31-2002, 07:07 PM
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Ed_L Ed_L is offline
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I've always associated gamma with the midtones, but I never saw a technical explanation for it. Excellent choice for a term to discuss.

Ed
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Old 01-31-2002, 07:35 PM
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Moving the mid-tone values without changing the shadow or highlights would be the equivalent of increasing the slope of the straight portion in the middle of the S-curve. This middle portion is usually the longest section, and therefore is usually used to define the gamma as a single number for simplicity's sake.
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Old 02-02-2002, 04:30 PM
Mike Mike is offline
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In wet style photography, the Horz. and Vertical values are reversed for negatives, with the H being the amount of light getting to the film, and the V being the density of the film AFTER IT IS DEVEOPLED. We refer to it as a DlogE curve (Density / log of the Exposure). I think I remember this coming from a couple of guys called Hurter and Drifield (watch that spelling). They were from England I believe.
We used to use a thing called a "reproduction cycle" to set up for film copy work that projected exactly how to set up the equipment so we could make copies based on the gamma of the orginal photo, the gamma of the copy negative, the gamma of the printer optics and the gamma of the printing paper. It would work about 95% of the time.
Remember that scenes have gamma too. A brite sunny day with deep shadows and very brite highlites is a day with a high gamma, while a cloudy overcast day with hardly no highlites and not really any dark shadows is a low gamma day. (those low gamma days are quite common were I live!!!). We would change the film developing time to add to or delete some of the contrast produced by the gamma of the scene.
Anyway, just thought I would add this for whatever background info you wanted. Thanks for reading and I do find this new feature quite helpfull.
Mike
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Old 10-21-2002, 11:19 AM
Emjai Emjai is offline
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Is this related to levels? I always use levels to tweek digital photos, but I've never really understood how to use curves.

Malcolm
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Old 10-21-2002, 11:26 AM
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Moving the midtone slider in levels would be a rough analogy to increasing/descreasing gamma, though with very little fine control.
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Old 10-22-2002, 09:50 AM
Stephen M Stephen M is offline
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If the internet has an 'official' (majority peer review accepted) FAQ on gamma - then this is it:

http://www.inforamp.net/%7Epoynton/GammaFAQ.html

http://www.inforamp.net/%7Epoynton/n.../GammaFQA.html

http://www.inforamp.net/~poynton/not.../GammaFAQ.html

http://www.inforamp.net/~poynton/not...ning_Timo.html

Sincerly,

Stephen Marsh.
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Old 10-22-2002, 10:01 AM
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Timo's views on gamma-correction are very controversial, and even contradicted by Adobe, but I'm sure he knows plenty about gamma in general.

This is not the place to get into a pro or con of his views, since megabytes of discussion on this already exist on usenet (use google to search on his name to read for yourself).

Just be aware he is considered by many to be a guru, but many more (including just about all the other gurus) discount his theories as "eccentric" (putting it nicely).
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Old 02-02-2004, 01:08 AM
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Quote:
Originally Posted by Doug Nelson
Gamma is a display of the contrast of an image represented by a graph. Traditionally, the vertical axis represents the input values, with the lowest point being the shadows. The horizontal axis represents the output values, with the leftmost point representing the shadows.
I just found this section! Believe it or not. (This site is big) I don't want to step on anyone's toes here, but ... that definition at least doesn't correspond to the RGB Curves in Photoshop.

In Curves in Photoshop the axes are opposite of what you write here. The X axis are the input values and the Y axis represents the output values. You can see this for yourself in the Input/Output boxes, and you can experience it visually: if you "lift" the curve, darker midtones become lighter.

So by lifting the curve, you get "more for less". If you're adjusting RGB, you get more light. If you're adjusting CMYK (reversing the axes) for output, you get more ink. Isn't this correct?
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