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Histogram
Many digital cameras have an histogram that is viewed on the LCD or electronic viewfinder. It indicates whether or not an image is properly exposed. The histogram shows if an image is too dark (underexposed), too light (overexposed) or if the exposure is just about right. 
The distribution of light and dark in an image is displayed on the histogram. Darker areas are shown to the left and bright areas to the right. A good exposure is indicated when mid-tones display approximately halfway between the darkest and brightest tones.
If the histogram shows that an image is not properly exposed, change exposure settings and retake the picture. If an image is too dark, adjust exposure compensation to the plus side. It it is too light, adjust exposure compensation to the minus side. For digital cameras without manual settings, lock exposure on another part of a scene, then recompose before shooting.
Many digtial cameras have a "live" histogram. Actual hanges in exposure can be seen on the histogram while framing a shot. Other cameras have histograms that can only be viewed in playback mode after an photo is taken.
Advanced photo editing programs have histograms. They serve as a guide during the editing process when adjusting the tonal range of an image.
