Gamma is an adjustment factor used by computer systems when
converting image pixel brightness values to monitor
pixel brightness values. Gamma can also affect color, but this discussion is limited to brightness.
A computer system
takes electricity and uses it to reproduce an image or text on its
monitor. Every image pixel has its own brightness value, which will be referred
to as the input brightness value. A computer system takes these input
brightness values and converts them into output brightness values for its
monitor. The brightness at which pixels are displayed on a monitor is a
nonlinear relationship between the input values and the output values called the
brightness response curve. The response curve formula is expressed as output
brightness equals input brightness to the power of an adjustment factor. This
adjustment factor is called gamma. Therefore, output brightness = input
brightness ^ gamma. In order for an image pixel of medium tone to appear medium
tone on a monitor, its input brightness value must be adjusted by the gamma
Because input brightness values range from 0 to 1 inclusive, the higher
the gamma value, the lower the output brightness value. For example, 0.5 to the
power of 1.8 equals 0.287. But 0.5 to the power of 2.2 equals 0.218. As you
can see, the higher gamma factor resulted in a lower output brightness value.
The two common gamma values you will read about are 2.2 for Windows based computers
and 1.8 for Mac computers. Because of the differences in gamma, an image
optimized on a Windows based computer will appear brighter on a Mac computer
because the Mac output brightness values are higher than they are on a Windows
based computer. And an image optimized on a Mac computer will appear darker on
a Windows based computer. For a Levels adjustment that will correct for
computer platform differences, please visit the
You may have heard that gamma only affects mid tones.
This is partially correct. It is more accurate to say the closer a tone is to
mid tone, the more it will be affected by gamma. This is due to the nonlinear
property of the brightness response formula. If solid black has an input value
of 0.0 and solid white has an input value of 1.0, then 0.0^1.8 = 0 and 0.0^2.2
is also 0. And 1.0^1.8 = 1 and 1.0^2.2 is also 1. Therefore, solid blacks and
whites are not affected by gamma. Highlights and shadows are minimally affected
and mid tones are affected the most.
Some of you may have noticed that this
definition states that gamma values range from 0 to 1 but the gamma slider in
the Levels adjustment ranges from 0.10 to 9.99. The gamma slider in Levels
shows the relative change from the original brightness value to the new. The slider
value does not show the actual gamma value or the internal brightness value.