Digital cameras work out the correct exposure by measuring the light levels across the scene you’re photographing. But they can split the scene up in different ways to make these measurements, using different metering patterns, or metering modes.
By default, cameras analyse the scene by splitting it up into segments. This is called ‘multi-segment’ or ‘multi-pattern’ metering, and the camera is looking for patterns of brightness distribution to try to identify the kind of subject you’re photographing, which will help it work out the correct exposure.
The name for this kind of metering changes from one maker to another. Canon calls its system ‘Evaluative’ metering, while Olympus calls it’s multi-pattern system ‘ESP’ metering.
Normally this works very well. Sometimes you might need to apply a little exposure compensation for particularly difficult lighting, but many photographers use multi-pattern metering exclusively.
The trouble with multi-pattern metering is that it’s difficult to second-guess what it’s going to do. For novice photographers this won’t be a problem because the camera is likely to make a better guess at the correct exposure than they will. But more experienced photographers may prefer to switch to a simpler metering system for some shots. This sounds paradoxical, but simpler metering modes can be used and interpreted much more selectively.
This is why almost all cameras have a center weighted metering mode. In this mode, the camera measures the average light value across the whole scene but adds extra emphasis to the center of the frame. Center weighted metering is cruder that multi-pattern metering, but can work well for high-contrast scenes where the response of a multi-pattern system can be unpredictable.
Cameras also offer a spot metering mode. This is a much more precise system that takes a reading from a small area of the scene only, sometimes just at the center of the frames, sometimes under the active focus point (depending on the camera). Spot metering is very useful if you want the whole exposure to be based on a single small area, such as a spotlit stage performer.
Canon offers an interesting ‘partial’ metering mode on some cameras. This is like a ‘softer’ spot metering mode that covers a wider area. You can think of it as being half way between center weighted metering and spot metering.
Finally, some more advanced cameras offer an average metering mode that’s the simplest of all and harks back to the days when cameras had a single photocell. Average metering is heavily influenced by bright areas or light sources, but some photographers like its simplicity and predictability.