Metering modes change the way a camera determines its exposure, offering valuable flexibility when not using full-manual mode. In fact, a metering system is used whenever a camera must automatically determine any exposure-related parameter: shutter-speed, aperture, auto ISO and even flash output.
The most common metering mode in modern digital cameras is called by many names: matrix, evaluative or multi-segment. It involves measuring the brightness of different parts of the image and comming up with an exposure to suit the overall picture. While this mode is generally the safest, it is also the hardest to pre-visualize, specially on display-priority LCDs and most DSLR cameras which lack a live-view.
Other useful metering modes are: average, center-weighed, spot, highlight-spot and shadow-spot. Unless you work exclusively in full-manual mode, having these alternate metering modes can be valuable.
Average metering exposes a scene so that its average brightness corresponds to a mid-tone value, frequently called 18% grey. It does not leave much user control, but its results are more predictable than multi-segment metering. For this reason, this mode is less valuable than other metering modes.
Center-weighed is basically a two-segment metering mode. This mode generally gives an acceptable exposure but its results are hard to predict since what consists the center of the image and its importance are not well-defined. Basically, a center-portion and an outside-portion is separately metered for averega exposure, then an exposure is chosen between those two that is closer to the center-portion metering than to the outside-portion metering.
Spot metering is probably the most professional metering mode. It meters off of a small portion of the image, between 1% and 5%, usually at the center or at the focus point. Whatever subject is in the metering-spot will be rendered as a mid-tone. As such this mode provides the greatest control and the greatest possibility of error. Using spot-metering involves chosing the mid-tone value of a photograph, which isn’t as easy as it sounds. Since nothing else is considered, incorrectly choosing the metering-spot can lead to greatly under-or-over-exposed areas. The user-control given by this mode makes it the most useful and valuable metering mode. Even though it is a fairly common mode, it is not even present on all DSLR cameras.
Both highlight-spot and shadow-spot metering modes are uncommon variants of spot-metering. These two modes work just like spot-metering but consider the metering-spot as either a highlight or a shadow, respectively. The rationale of these modes is that it is easier to recognize highlights and shadows than it is to recognise a mid-tone. Needless to say, these are quite useful modes but are only present on a handfull of cameras such as the Olympus E-500 and E-330.