Ideally you’d like to have detail in both the highlights and shadows: nothing washed out, nothing completely black, and a histogram that shows no pixels pushed up against either the right or left edge, as in this photo of Lundy Canyon.
The most important part of the histogram is the right-hand edge, because that’s where the highlights are. If you see pixels pushed up against the right edge of the histogram, or a spike like this, that means some pixels are overexposed, and parts of the image are washed out.
If there are pixels pushed up against the left edge of the histogram, that means parts of the image have gone completely black.
Often the scene has too much contrast to retain detail in both highlights and shadows. Then you have to choose: would you rather have detail in the highlights, and let some shadows go black? Or would you rather keep detail in the shadows, and let the highlights wash out?
The answer depends on the image. Which are more critical, the highlights or shadows? Here’s a clue: in 99 percent of landscape photographs, the highlights are more important.
Why? First, your eyes are drawn to highlights, so compositions work better if the key areas are bright. If the main subject is dark you’ve got a problem. (Of course there are exceptions, like silhouettes.)
Second, in real life we never see things that are washed out. We can always see detail in bright spots (except when looking at the sun itself, or the sun reflected in water or glass), but we can’t always see detail in dark areas. So it seems unnatural to view something in a photograph that looks washed out, yet it feels perfectly normal to see regions of pure black.
So if you can’t keep detail in both the highlights and shadows of an image, 99 percent of the time you should sacrifice the shadows and keep the highlights.
The left histogram represents an image with black shadows, but detail in the highlights. The right histogram shows a photograph with detail in the shadows, but washed out highlights. In most landscape photographs highlight detail is more important, so it’s better to see this left histogram than the right one.
So if you take a photo, and the exposure is off — pixels are pushed up against the right edge of histogram, for example — how do you fix it? If you're in automatic mode, like Program, Shutter Priority, or Aperture Priority, you have to use your exposure compensation dial. In Manual Mode you can change either shutter speed or aperture to lighten or darken the exposure. Maybe I'll explain the details in another tip!
