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A metering system that uses several sensors to take readings from various sections around the image and calculates an exposure based on these values. This type of metering, also known as matrix or segment, is better for difficult lighting situations, such as backlit subjects, as it can determine when several sections are brighter than the area that you have focused on and adjust to compensate. Some of the more sophisticated versions couple up to the focusing system to offer even more precise exposure. Basic systems have as few as two metering zones where advanced ones can have eight or more segments.
Cameras and exposure meters can take readings of the light levels in a number of ways, from basic to advanced methods. The most basic is Centre-weighted (CW) or average metering that takes a measurement from most of the image area. A more sophisticated version of this is Partial (P) metering that has a narrow area of measurement that is still based on the centre of the image and Spot (S) that can measure from as little as 1 of the image. The most commonly used in more advanced cameras now is Matrix, also known as Multi-pattern (MP) or segment metering, that takes readings from several parts of the scene and produces a calculated average. Buying advice In most cases the basic metering with an auto camera is fine, especially if you have the sun behind you or it is overcast. But when you start to try more advanced shots, such as the sun behind the subject for a backlit halo effect or an archway with light streaming through the pillars or a spotlit subject against a dark background, you may find the standard meter will let you down. If you can see yourself shooting subjects like this, buy a camera that lets you switch over to spot or multi-pattern metering.