A histogram is a pixel map. It shows how dark and light pixels are distributed within your photograph. This image of a redbud is low in contrast, with lots of medium tones, so the histogram shows a mound of pixels in the middle. The image of gulls on a pier has no medium tones; it’s dominated by the light gold water, with the contrasting dark areas of the gulls and pier. The histogram shows a big spike on the right side — that’s the water. There’s a smaller spike on the left — the gulls and pier. Both images are properly exposed. They’re just different photographs, and the histograms reflect that.
Left, Right, and Middle
Reading Histograms
 
Exposure used to be the single most difficult technical problem in photography, but digital cameras have made this thorny issue much easier. Does that mean you can now just turn on Program mode and turn off your brain? Sorry! Thought and care are still required. The basic problems of exposure have not changed. The only difference is that you can see right away whether you got it right.
This doesn't mean judging the exposure by the camera's little LCD screen; these are notoriously unreliable. It means using that invaluable tool, the histogram. There's a lot of confusing, conflicting, and inaccurate information out there about histograms, but reading them is simple.
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