Histogram
A graph that offers a quick reference to a picture's exposure and contrast settings. It creates a map of the distribution of tones, beginning with the darkest on the left and progressing to the brightest on the right. The scale goes from 0 (complete darkness) all the way up to 255 (complete brightness), and the height of the graph at any one position shows the relative number of pixels in the image that have that brightness level. The general form of the histogram provides an idea of the tonal range of the image as well as the existence of any clipping at a look. You may increase the image's contrast and exposure by adjusting the form of the histogram using tools like Levels, which are available in most image editing programs.
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