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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 factor.

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 Downloads page.

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.