A histogram is a graph that shows the distribution of intensities in an image. The horizontal axis represents the full range of tonal values, the vertical axis indicates the number of pixels for each intensity value.

The histogram can provide an indication of the image key type. A low-key image has a greater number of pixels in the shadow area; a high-key image has more pixels in the highlights; a mid-key image has detail concentrated in the midtones. An image with full tonal range has a more uniform number of pixels in all areas.

These figures show a scanned image on the left and its histogram on the right.

An image with uniformly spaced tone values

An image with more highlight tone values (high-key)

A image with more shadow tone values (low-key)

An image with more midtone values (mid-key)

A mid-key grayscale image