Automatic exposure is a standard feature on all digital cameras. The metering system measures the amount of light in a frame and determines the best exposure. Many cameras have more than one metering mode and each evaluates a scene in a different way.
Center-weighted is the metering system of choice on compact digital cameras that do not offer other metering modes. Currently it is the most common metering system.
Exposure metering is averaged over the entire frame with emphasis placed on the central area. Center-weighted metering is used for general and portrait photography, when it’s important to have the central portion of the scene properly exposed.
Matrix (evaluative) metering
A complex metering system whereby a scene is split up into a series of zones. Overall exposure is based on evaluating each zone individually and taking an average of the total light readings. After the light is measured, the camera determines the best exposure.
Matrix metering is effective for scenes and subjects that have even lighting.
Spot metering covers just under four percent of the viewfinder area. It takes a precise exposure reading only at the very center of the frame and disregards the rest.
A spot meter is used when there are extremes in the brightness of a scene (high contrast scenes), such as when a subject is back lit or has bright light on it and the background is dark.
Spot metering can also be useful for close-up and macro photography.
Partial metering is similar to spot metering but covers a larger area of the viewfinder, about 13.5 percent. It is useful for taking portrait photos when the subject is back lit.
Partial metering is also often used when a scene has bright or dark areas around the edges of a subject. Underexposure is minimized by metering on the main subject in a scene.
Both spot and partial metering are considered advanced settings. They give the photographer more control over exposure than do matrix and center-weighted metering modes.