What does “the visual system imposes a power function of about 1/1.25” mean in gamma correction?
Asked 6/24/2023
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In Charles Poynton’s discussion of gamma, I’m confused by the statement that “the visual system of the viewer imposes a power function with an exponent of about 1/1.25.” Does this mean that when we look at a display, our perception always follows a brightness curve of 1/1.25? I’m trying to understand whether this is a fixed property of human vision or just an empirical approximation used in display imaging.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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If you read a little closer, it says that when viewing a CRT which is gamma corrected to 2.5, so that it "reproduces physical luminance correctly," the viewer subjectively prefers a 1/1.25 gamma correction in the other direction for increased contrast; resulting in a gamma 2.0 curve as being optimal for a CRT... subjectively and situationally variable.
It goes on to explain that 2.2 is the standard and how it relates (falling in-between 2.5 and 2.0).
In terms of the visual system, a human requires a log2 (2x) change in luminance to be perceived as an equivalent change... which is why photographic exposure is logarithmic. But, of course, human vision and preference does vary by individual; which makes the whole gamma correction thing a bit arbitrary.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
3y ago
0
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It does not mean your eyes always force every display to have a fixed 1/1.25 brightness curve. In this context, it means human visual perception is often modeled empirically as a nonlinear response, and one convenient approximation is a power function with exponent about 0.8 (1/1.25).
The key point is that gamma curves are engineering models fitted to experiments with human observers, not laws derived from first principles. Poynton’s discussion is about how display output and human perception interact: for example, if a CRT reproduces physical luminance with one gamma, viewers may still prefer a somewhat different overall system response because it looks better subjectively, often with slightly higher apparent contrast.
So 1/1.25 is best understood as an approximate perceptual compensation term, not a universal constant. It reflects typical viewing conditions and average observer preference, and it can vary with display type, scene content, adaptation, and the individual viewer.
In short: it’s a perceptual model used to explain why a display system’s “best-looking” gamma may differ from strictly physical luminance reproduction.
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