Use AE lock when the area of focus is to be different from the exposure metering area or when you want to take multiple shots at the same exposure setting. Press the <A> button to lock the exposure, then recompose and take the shot. This is called AE lock. It is effective for backlit subjects.
AE LOCK EFFECTS
When the lens' focus mode switch is set to <MF>, AE lock is applied at the center AF point.