# What does the Gamma histogram represents?

I do not understand what does the Gamma Adjustment histogram represents, as it differs a lot from the regular brightness histogram. Would very much appreciate an explanation :)

Here taken from Canon's Digital Photo Professional:

• it might be just the same thing scaled differently, but i don't have a proof... – szulat May 6 at 15:59

I found the answer I was looking for here: https://www.cambridgeincolour.com/tutorials/histograms2.htm

Basically, it says that the RGB histogram displays the summation of the three Red, Blue, Green histograms, while the Gamma histogram displays the luminosity histogram, which gives different weight to different colors, as the human-eye is more sensitive to green than red.

A tried and true method used to analyze an image is to measure various places (densities or intensities) and then, using graph paper, plot these measured points. When finished, the plot resembles ½ of a bell curve. We divide this curve into regions. The lower portion is called the “toe”. This plot graphically displays that the image is sluggishly starting to form. Next is called the region of the straight line. This portion of the graph portrays graphically a proportioned response to the exposing light. We can measure with a protractor the angle of the straight line. If the image contrast is low, the angle of the straight line is depressed. If the image displays high contrast, the straight line show this fact; its angle will measure 45⁰ or greater. Historically we covert this angle using trigonometry to a tangent (TAN). Most images with adequate contrast have a straight line with an angle of about 40⁰. The Tan of 40 = about 0.8. Traditionally this is the target contrast for images that display respectable contrast. We are talking about the science of sensitometry -- the making of test exposures, and the science of densitometry -- the measuring of photographic images. It is customary to name the TAN of the angle of the straight line “gamma”.

The graph that follows is for film -- digital images are a subset of film when it comes to making determinations; we graph these also.

35⁰ = gamma 0.7 = flat 45⁰ = gamma 1.0 = contrasty