The histogram function can be used to make image adjustment less subjective.

The histogram shows the distribution of brightness of the pixels in an image as a graph, with black on the left and white on the right.

Most images have a wide tonal range, so the histogram extends continuously from black on the left to white on the right..

This picture of the National Cathedral in full sunlight has lots of light tones, so the histogram is peaked on the right.

This picture of stonework has moslty gray values in the center of the histogram.

This picture of a tapestry is underexposed, so the histogram shows only a peak in the dark gray region on the left.