Using a Histogram
by Mike Schumacher
A histogram is the representation of values of light in an image. There are 256 values from 0 to 255 with 0 being pure black and 255 being pure white. It's like a plot; values run from black on the left to white on the right. The higher the display from bottom to top, the more of that value. Middle gray is in the middle of the graph.
When you look at a histogram and see the display bunched up against the left side, your image contains a lot of dark light values and is most likely under exposed. Likewise, if it's bunched up against the right side, you are most likely over exposed and have blown out the highlights. Most of the time, you want a display that is not bunched up against either side. This will allow open shadows and detail in the highlights.
A histogram is one very good way to adjust exposure in the field and in the digital darkroom. Most digital cameras and image editing programs have a histogram.