What is a histogram and how do you use it?

A histogram is a graph depicting the distribution of brightness values in the image. The horizontal axis shows the brightness level (from 0 to 255), with darker pixels towards the left side and brighter pixels toward the right. The vertical axis shows the proportion of pixels at each level of brightness. By examining the histogram you can gauge the over-all exposure of an image. When the histogram is higher on the left side, the image is mostly composed of dark pixels, causing it to appear dark-either because the image is underexposed or because it is a dark scene such as night shot or sunset. When the histogram runs off the right side, the image will have many white pixels-either because the image is overexposed or because it is a bright scene such as the beach or snow. A histogram with a fairly even distribution of pixel values usually indicates that the image is well exposed, with good contrast. However, the histogram distribution of a well-exposed image will vary greatly depending on the subject.