Understanding Gamma for Photography

[Marc Hildebrant]

Group,

I have a "fuzzy" understanding of the use of Gamma in Photography. First, my understanding is that this Greek symbol is a power of a function used to convert pixel luminance values to a different value. The original idea relates to the non-linear performance of CRT (Cathode Ray Tubes). Appling the math function "in reverse" corrects the non-linearity, thus giving an overall linear mapping from sensor output to display as seen by the user. So far, so good.

Often, though, gamma gets talked about to describe the human eye's response with the idea that this is also used with the gamma function. At this point, it's use becomes confusing, as my understanding is that the camera output to the eye should be linear. Thoughts?

Marc

 

[smoke665]

A simple definition- it defines the relationship between a pixel's numerical value and its actual luminance. Cambridge goes into more detail. https://www.cambridgeincolour.com/tutorials/gamma-correction.htm

 

[Marc Hildebrant]

smoke665,

Thanks for the link and I did read it, however, I still have some questions.

The article refers to using the gamma function for both overall correction for the display's properties and for compensating for the human eye's sensitivity to levels of light.
Other sources state that the gamma function's purpose is to only correct the overall image path so that the camera's output as displayed on the monitor is a linear function.

Tone curves I can understand, but it seems that the overall process from taking a picture to displaying it should be linear, not changed via a non-linear gamma function.

Comments?

Marc

 

[smoke665]

it seems that the overall process from taking a picture to displaying it should be linear, not changed via a non-linear gamma function.

That was explained in the article I referenced. The eye does not perceive luminance in a linear manner. In a linear model twice the light on a pixel would double the luminance, but twice the light on the eye is only perceived as fractionally brighter, with more sensitivity in dark tones and less in light. Gamma redistributed tones to more closely match the human eye's ability to perceive. As a footnote, this is why it's necessary to soft proof your images prior to print, otherwise highly saturated colors will be out of gamut on a print. Most of the labs use sRGBIEC61966-2.1and some will supply their own proprietary color profile.

 

[Marc Hildebrant]

Adding to the info sighted by smoke665 is some from:

A quote:
This is where gamma comes in. Instead of a linear OETF, we take the RAW data and we apply a non-linear OETF (the gamma curve) that amplifies the tones much the same way the eye would. The result is the details in the image that originally had tones within the eyes’ sensitivity range are now outside that range and occupy some of brighter tonal ranges. Now, when we do our binning, the same as before, details that had been missed with a linear OETF, are picked up.

Press enter or click to view image in full size

When the resultant image is presented to the display, inverse gamma (EOTF) is applied, which undoes the brightening that was done after gamma application. The result is that the image returns to its original luminance level but with far more detail in the darks/shadows region.

 

[tinytmp3 2]

Group,

I have a "fuzzy" understanding of the use of Gamma in Photography. First, my understanding is that this Greek symbol is a power of a function used to convert pixel luminance values to a different value. The original idea relates to the non-linear performance of CRT (Cathode Ray Tubes). Appling the math function "in reverse" corrects the non-linearity, thus giving an overall linear mapping from sensor output to display as seen by the user. So far, so good.

Often, though, gamma gets talked about to describe the human eye's response, with the idea that this is also used with the gamma function. This concept is often discussed in resources like Jose Soriano Photography, where the relationship between perception and gamma correction is explored. At this point, its use becomes confusing, as my understanding is that the camera output to the eye should be linear. Thoughts?

Marc

Gamma is a nonlinear function originally used to correct the nonlinear brightness response of CRT displays. The display’s light output was proportional to the input voltage raised to a power of about 2.2, so images were encoded with the inverse function to make the overall system appear visually linear. Although modern displays no longer have this issue, gamma encoding is still used because the human eye responds to brightness nonlinearly, being more sensitive to dark tones than to bright ones. Applying gamma makes better use of limited digital values and ensures that images look perceptually correct, even if the actual light levels are not physically linear.