The A (L) Button
Press the A (L) button to lock focus and exposure.
Autoexposure (AE) Lock
Pressing the A (L) button locks exposure at the current setting. AE lock can be used to recompose shots after metering a subject that will not be in the selected focus area in the final composition, and is particularly effective with spot or center-weighted metering.
Press the A (L) button to lock focus on the current subject when AF-C is selected for focus mode. When using focus lock, choose an AF-area mode other than auto-area AF.
Locking Focus and Exposure
Follow the steps below to use focus and exposure lock.
Set focus and exposure.
Position the subject in the selected focus point and press the shutter-release button halfway to set focus and exposure.
Lock focus and exposure.
With the shutter-release button pressed halfway, press the A (L) button to lock both focus and exposure (an AE-L icon will be displayed).
Recompose the photograph and shoot.
Focus will remain locked between shots if you keep the shutter-release button pressed halfway (AF-S) or keep the A (L) button pressed, allowing several photographs in succession to be taken at the same focus setting.
Do not change the distance between the camera and the subject while focus lock is in effect. If the subject moves, focus again at the new distance.
When AF-S is selected for focus mode, or if the camera is shooting using AF-S when AF-A is selected for focus mode, the focus will lock while the shutter-release button is pressed halfway (focus will also lock while the A (L) button is pressed).
If On (half press) is selected for Custom Setting c1 (Shutter-release button AE-L), exposure will lock while the shutter-release button is pressed halfway.