There are three methods your camera has to meter light:
Spot Metering: The camera meters the light indicated by the spot. Use this when there is a relatively small object within your scene which you need to be perfectly exposed. Assures the subject will be correctly exposed in “difficult lighting situations” even though the background is too bright or too dark.
Centered Weighted Metering: The camera meters the entire frame but assigns greatest value to the area indicated by the circle. Use centered weighted metering when there is a wide range of light within the frame by metering off the area you want correctly exposed. This is the classic metering technique for portraits.
Matrix Metering: The camera meters the entire frame then sets exposure for the best overall image. This is used in most situations.
By Steve Guymon