CENTER-WEIGHTED AVERAGE METERING
Center-weighted average metering should be considered the normal metering mode for most picture-taking situations. When the metering selector is set to "AVERAGE" position, exposure is based on an average of the various light values in the scene with additional emphasis (weight) given to the center area. This produces consistent, reliable results in most situations with a minimum of effort.
Use of AE lock
This control is used in P, A, or S autoexposure (AE) mode to hold a meter reading for the main subject while you recompose and release the shutter. AE lock should be used when the main subject is not centered in the frame or when the brightness level between subject and background is very high, such as for backlit or spotlit subjects. To use:
Shift camera´s position so subject fills most of frame. For smaller subjects, you may need to move closer (or zoom in).
With meter on, press and hold AE Lock (marked AEL).
While keeping it pressed, recompose picture, focus, and release the shutter.