The metering systems of today’s digital cameras are light years beyond cameras of just 10 to 15 years ago. What this means is that in many situations, the camera’s meter, left to its own devices, is going to do an excellent job at getting a good exposure. While evaluative (or matrix) metering and center-weighted average metering take into account the entire scene, albeit in different ways, spot metering mode provides a tool for metering only the part of the scene you as the photographer consider most important.
Spot metering is especially helpful when the subject is much brighter or darker than the background, and the subject does not make up a majority of the image. Most cameras, when set to spot metering, are set to measure an area of the image as small as 1.5% of the total image area. This varies from camera to camera, so check your manual for the exact specification. In addition, while most consumer cameras use the center of the image for spot metering, usually defined by a circle in the viewfinder, others allow for tying spot metering to the active AF point, which then ties your choice of focus point to the meter.
Generally, if you’re spot metering, and your camera’s spot meter is at the center of the image area, you will have to use Exposure Lock as well. This is because if the meter only works at the center of the image, and the area you’re metering is NOT the center of the image, you will need to recompose after metering. Exposure Lock ensures that your exposure is locked in once you have metered the scene, before you recompose the shot. Exposure lock is generally activated by pressing a button on your camera while metering. It will then hold the exposure you locked in, at least until you release the shutter button. Some cameras will hold the metered reading until you deactivate Exposure Lock. On Canon cameras, Exposure Lock is achieved by pressing the (*) button. On Nikons, exposure can be locked by pressing AE-L.
When you’re dealing with a high contrast situation, such as a bright background with a dimly lit subject, or a dark background with a brightly lit subject, switching your metering mode to Spot Metering can be an easy way of ensuring that your exposure is exactly where you want it.