That really depends on a few things.

It depends on how much of the frame your camera meter is reading. Some cameras average meter values across the whole image, others use center weighted averaging, and others measure just a small area in the center (spot meter). Many cameras can be set to choose one of those three options and many advanced cameras allow you to pick the area within the frame that will be metered.

Once the camera reads the scene, you have to remember that the camera wants to make the metered area a middle tone. So if it reads a solid black object, it will tend to overexpose to make it gray. A solid white object will tend to underexpose to be a light gray.

If your subject is large enough in the frame and middle toned, it generally will influence the meter enough that compensation isn't necessary. However, if the subject is small and the background occupies too much space, the meter will adjust to the background. You may therefore need to overexpose against white backgrounds and underexpose against dark backgrounds.

The beauty of digital is you can take a shot, look at the image (or the histogram) and see if the exposure was correct. If not, adjust your exposure and reshoot.
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Chris Harrison
San Antonio, Texas

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