To many of us using digital cameras, the inner workings of the camera are like a black box- press the button, and the camera quietly, unobtrusively does what it does best and takes a quality picture. Understanding how it does what it does, however, can help you to get the best out of your camera when it is out of its depth. One such scenario is in the case of camera metering. Metering is the camera’s way of trying to optimize exposure, and, like autofocus, there are a number of different strategies or “modes" available to you in order to tailor the metering to your particular shooting conditions.
While your camera is quite excellent at detecting light, it is not yet capable of determining what that light means, where it is coming from, or what, specifically, you’re trying to take a picture of. The most common default metering mode takes a weighted average of the light levels across the entire scene; in most cases, this setting will probably give you the results you want, as long as the shooting conditions aren’t too extreme.
If there is a very strong variance in light levels across different portions of the scene, you may want to consider using a spot metering mode that only evaluates levels in the center of the scene. Be warned, though, that spot metering can sometimes throw the baby out with the bath water by sacrificing the rest of the scene to get the desired results in the center.
As a compromise between the two, center-weighted metering prioritizes the center but still tracks lighting levels in the surrounding region. It attempts to arrive at a happy medium that allows both the center and the extremities to be properly exposed.
Under backlit conditions, it may be difficult or impossible to insure that the background is properly exposed without also making the subject too dark- this is where fill-in flash can be extremely handy. Another potential way of dealing with such situations would be to resort to high dynamic range (HDR) photography, a technique that allows you to blend two exposures to get a wider range of detail overall.
For a more in-depth look at your camera's metering modes, please click here.