Background problems

Its not just the subject of your photograph that can give you exposure headaches. The tone of the background is just as important, and can have a big influence on the exposure reading. Even if your subject contains fairly even mid-tones, an unusually light or dark-toned background can produce exposure errors; the scale of the error will depend on how much of the frame is taken up by the background. It can also be hard to judge exactly how much emphasis the camera is giving to the subject itself, since multi-pattern metering systems may concentrate on the object in the middle of the frame, which may or may not be where your subject is.

You may not encounter completely black backgrounds like this when you're out shooting, but dark tones will have the same effect.

Light backgrounds cause mid-toned subjects to underexpose if they are not big in the frame. Bear this in mind when framing people against pale skies.

Size matters

The images below feature a mid-tone subject set against a dark background, but shot at two different zoom settings, so that in one picture the onions and ginger take up nearly all of the frame, while in the other theyre quite small relative to the background. In both cases the cameras default auto-exposure settings were used; the close-up shot is correctly exposed, but in the zoomed-out version the larger area of dark background has fooled the camera into overexposing by 1.3 EV. We tried a similar experiment using a light background. By zooming right in on the artificial fruit, we excluded nearly all of the background, and the resulting exposure was pretty well spot-on; when we zoomed out,however, the proportion of the frame taken up by the background was far higher, leading the camera to reduce the exposure by 1.3 EV, which left the shot underexposed. The degree to which the background influences exposure will depend on the amount of the frame it takes up, and its brightness, but it can make a big difference.