Current DSLR cameras have some pretty sophisticated metering systems giving you amazing flexibility in setting your exposure controls. The specific light meter options depend upon the model of your camera but most cameras today offer some, if not all, of these metering modes.
[Note about the graphics below: The points indicated by the etchings in the viewfinder are focus points. Your metering sensors do not necessarily correspond with the focus points. These are intended to illustrate the basic metering patterns.]
Matrix / Evaluative
This is generally the camera’s default or standard metering mode. It is the most intelligent mode in that the camera will process the meter readings using complex algorithms in an effort to determine the optimum exposure. Although each manufacturer – and often each model – may use different calculations, this mode usually does a very good job of determining the best settings.
One important note is that when using this metering mode, the exposure is based upon the assumption that the main subject is at the position identified by the auto focusing system. Other metering patterns don’t make this assumption.
Also, as a side note, it is most often the evaluative meter that is used to determine the flash exposure, regardless of the metering mode you use for the ambient light exposure.
Like the Matrix/Evaluative meter, all the sensors are used in Center Weighted mode. When the exposure is calculated, the sensors in the center of the viewfinder are weighted more heavily than the outer sensors. You use this mode when the subject is centered in the viewfinder.
When in Partial metering mode, only the center sensors are considered when calculating exposure settings. The area covered by partial metering is generally about 10% of the viewfinder and is always in the center of the viewfinder.
Partial metering is a feature used by Canon for many of their consumer level SLR cameras.
This is for metering a specific part of the subject or scene and the area covered is only about 2 – 4% of the viewfinder. By default this area is in the center of the viewfinder but some cameras allow you to move the spot to other locations. Serious photographers love the accuracy and control offered by a spot meter. Properly used, a spot meter can help you understand the dynamic range of a scene and can help you pinpoint exposure settings for a specific subject or object.
As a general rule, I switch between the evaluative mode (Canon - Matrix with Nikon) and the spot metering mode. I find the accuracy and control of the spot meter to be very useful.