There are 2 uses for the Auto-Exposure AE lock:

  1. When you want to have the area of focus to be different from the area metered for the exposure.
  2. When you want to take multiple photos with the same exposure setting irrespective of the change in framing or light.

Press the <*> button to lock the exposure, then you can recompose and take the photo.The AE Lock locks the exposure for 4 seconds.

What gets measured will depend on the metering mode:

Metering Mode Point Used
Partial Center point
Spot Center point
Average Center point
Evaluative & AF point selected by camera AF point that has achieved focus
Evaluative & Manual selection of AF point AF point selected even if it didn’t achieve focus