The LCD Monitor provided on digital cameras may be useful to judge the subject’s composition or framing but it is not so perfect for judging the exposure. LCD Monitors use bright fluorescent light to reproduce the images and hence the photos tend to look brighter than what they actually are. Histograms in this situation help a lot to judge the exposure.

Histograms are graphical representations to verify the exposure of the image previewed on the screen. The graph represents the two extremes of the brightness of any photo: shadows on the left and highlights on the right. The starting point at extreme left shows zero level of light or pure black pixels whereas the ending point at extreme right shows the highest level of light, usually 255, the white pixels. So the brightness of tones goes on increasing progressively from 0 to 255. The height of the spike shows the distribution of that pixel in the image. Taller spike indicates more pixels of that tone. The graph indicates whether the image is too dark, too bright, or well-balanced. With an image having evenly distributed brightness, the histogram’s spikes and bulges will be clustered mostly at the centre. An underexposed image’s histogram will have a distribution of brightness that tends to be mostly on the left of the graph. An overexposed image’s histogram will have the bulge showing the brightness distributed mostly towards the right of the graph. All images may not show this characteristic of their histograms clustering mostly at the centre. Much depends on the subject of the image. In some cases, it might be appropriate for the histogram to show dominance at one end or the other, or both. For instance a correctly exposed photo of a model wearing white costumes if shot against a white background would be represented by a histogram that would show the spikes and bulges mostly on the right side, whereas histogram of a photo of full moon (with dark areas dominating the overall picture) would move towards left. For a subject that is composed of an average brightness the histogram will show the brightness mostly in the middle of the graph. The graph will be mostly on the left for an image dominating dark areas whereas the graph will be mostly on the right side for subjects that contains white dominance. Graph of a monotonous subject shows a straight, high and narrow spike. The graph broadens as the tones vary.

A few examples below demonstrate the behavior of the graph.

Clipping of Highlights: Some details in the highlighted areas are lost if the spike of the graph touches the right end of the histogram. This is known as clipping of highlights. Some cameras show highlight or overexposure warning on the previewed image. (A similar feature in Video Cameras is known as Zebra warning). The photographer now has to decide whether these details are important to obtain in the final image or not, if so under-expose to some extent and get details in the highlights.

Clipping of Shadows: Details in the shadows are said to be lost if the graph now touches the left end of the histogram. A few cameras do give warning for the shadow clippings too on the previewed image. Over-expose the photo if you desire to recover the lost details in the shadows.