Gamma correction is a method for controlling how midtones are rendered in a digital image. The effect of the gamma value, i.e. curve shape, on the original image is shown in the following images. In areas where the curve has been steepened the generated image has more contrast, in areas where the curve has been flattened the image has less contrast. Gamma correction requires a tradeoff, the contrast in areas of the generated image with a steeper curve will be enhanced while the contrast in areas with a flatter curve will be reduced.

Original image with no gamma correction

Increasing gamma steepens curve in shadows, adding detail

Decreasing gamma steepens curve in highlights, adding detail

Custom correction curve emphasizes extreme highlight and shadow detail

HP Scanjet gamma adjustment

The HP Scanjet application controls the gamma value via the Midtones slider, shown here at its minimum value.