Histogram

A graph that provides an instant guide to the contrast and exposure of a picture. It maps the distribution of tones, from the darkest on the left to the brightest on the right. The scale runs from 0 (solid black) to 255 (pure white), and the height of the graph at any point represents the relative number of pixels in the image with that brightness level. The overall shape of the histogram gives you an at-a-glance indication of the tonal range of the image and the presence of any clipping. You can use tools such as Levels in Photoshop to adjust the shape of the histogram and thereby improve the contrast and exposure of the image.