Definition: histogram

A bar graph that uses the width of the bars to represent the various classes and the height of the bars to represent their relative frequencies.

Camera Histograms
Digital camera histograms show the image's overall exposure. Using 256 vertical bars to represent brightness levels from 0 to 255, the leftmost bar is the darkest pixel level (0), and the rightmost bar is the lightest (255). The height of the bars represents the total number of pixels at that brightness level.

What is of most interest to the photographer is how the bars spread horizontally from left to right. For example, if there are no bars on the left, there are no black pixels in the image.

Light Pixels

Histograms appear on the same LCD screen used to preview the image. In this daylight example, there is an absence of dark shadows because the histogram shows no pixels on the left side (dark side).

Dark Pixels

In this example, the camera was pointed into a totally dark room, and the bars are confined entirely to the leftmost side (dark side).