Histograms

A histogram is the graphical representation of the tonal values of your image.

Understanding image histograms is probably the single most important concept to become familiar with when working with pictures from a digital camera.  A histogram can tell you whether or not your image has been properly exposed, whether the lighting is harsh or flat, and what adjustments will work best.  It will not only improve your skills on the computer, but as a photographer as well.

In general, an image that is underexposed will have a histogram with tonal values in the dark area to the left.

A correctly exposed image will have tonal values not too far to the left or right.

An overexposed image will have tonal values to the right in the light or very light range.

Tonal Range

The region where most of the brightness values are present is called the “tonal range.”  Tonal range can vary drastically from image to image, so developing an intuition for how numbers map to actual brightness values is often critical—both before and after the photo has been taken.  There is no one “ideal histogram” which all images should try to mimic; histograms should merely be representative of the tonal range in the scene and what the photographer wishes to convey.