The metering system in a digital camera measures the amount of light in the scene and calculates the best-fit exposure value based on the metering mode explained below. Automatic exposure is a standard feature in all digital cameras. All you have to do is select the metering mode, point the camera and press the shutter release. Most of the time, this will result in a correct exposure.
The metering method defines which information of the scene is used to calculate the exposure value and how it is determined. Metering modes depend on the camera and the brand, but are mostly variations of the following three types:
Matrix or Evaluative Metering
This is probably the most complex metering mode, offering the best exposure in most circumstances. Essentially, the scene is split up into a matrix of metering zones which are evaluated individually. The overall exposure is based on an algorithm specific to that camera, the details of which are closely guarded by the manufacturer. Often they are based on comparing the measurements to the exposure of typical scenes.
Center-weighted Average Metering
Probably the most common metering method implemented in nearly every digital camera and the default for those digital cameras which don't offer metering mode selection. This method averages the exposure of the entire frame but gives extra weight to the center and is ideal for portraits.
Spot (Partial) Metering
Spot metering allows you to meter the subject in the center of the frame (or on some cameras at the selected AF point). Only a small area of the whole frame is metered and the exposure of the rest of the frame is ignored. This type of metering is useful for brightly backlit, macro, and moon shots.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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