Digital SLRs utilize “through-the-lens” light metering; what you see through the viewfinder, you meter. TTL is reflective, measuring the amount of light bouncing off a subject. It’s accurate, and it means that subtle changes in composition—which can create dramatic changes in a reading—create instant adjustments to the exposure. This is great if you’ve set your camera on the ideal metering mode. If not, it’s a disaster.
Most SLRs offer variations on a few primary metering modes—spot, center-weighted and multi-zone metering. How they work dictates how they should be used.
Spot metering is simple. The light meter measures a small circle in the center of the frame. Usually, the spot measured is from 1% to 5% of the overall frame, so a subject silhouetted by strong backlight can be metered accurately—only the shadowed subject at which you’ve pointed the spot is factored in to the exposure.
Partial metering still places emphasis on the center of a scene, but the circle is larger. Center-weighted metering, however, blends this partial zone into an overall reading of the entire frame. The center is given emphasis, but the edges are considered, too.