The Histogram

The Histogram is a graphic representation of the exposure values for an image, and as such can be a very useful tool for the photographer seeking the best possible exposure. Most Digital SLR cameras, bridge camera and some compacts have a Histogram display available to view on the camera at the time of shooting, although this may need to be switched on in the camera menu.

The Histogram is made up of 256 bars or stacks, which run through through the levels of brightness found in the image. Stack 0 corresponds to black, stack 255 to white, with 254 levels of gray in between. The height of each stack is determined by the number of pixels that share that level of brightness.

Example Images

A well balanced histogram that shows a good range across the levels of brightness, with a good build up from 0 to 255 indicating that virtually all the image has good levels of detail across the entire range.

Under Exposed image. As you can see there are many pixels which have a 0 value, ie are black and have no detail, indicating that the shadows are "clipped". The fact that the whole histogram is over to the shadow sides indicate that a large number of pixels are dark, which would suggest under exposure.

Over Exposed image. Here we can see that the hilights are clipped, with many pixels having a value of 255, that is pure white with no detail. The fact that the majority of the pixels are indicating very light brightness values would indicate that the image is over exposed

Lacking Contrast. In this image we can see that although the curve of the histogram is in the middle, too many pixels are showing mid gray, indicating that the image is lacking in contrast and will appear as flat and gray, with few shadows and highlights.

Thus, using the Histogram function on the camera you can at a glance get a good indication of the exposure levels of the image that you have just taken, and make minor adjustments to enable the best possible exposure under the circumstances.