A histogram, in photographic terms, is a graphical representation of the tonal data contained in an image. Histograms are very useful in that, once a photographer learns to read them, they provide an infallible representation of an image’s exposure, contrast, and tonal range. If photographing in the field, the back LCD of an image is not to be trusted. The exposure depicted is usually either brighter or darker than the actual image being recorded will look when viewed on a computer, looks more contrasty, and is difficult to view in bright light on top of it all. Thankfully, you use the LCD to view the histogram and it will indicate the tones present in a graph. Histograms are also useful when processing photos because regardless of your monitor’s calibration or brightness level, a histogram again provides a mathematical representation.