The A Button
Press the A (AE-L/AF-L) button to lock focus and exposure for photographs taken using autofocus.
You can choose the role assigned to the A button. Use Custom Settings f3 [ ] and g2 [ ] in the Custom Settings menu to choose the roles played by the button during photography and movie recording, respectively.
Exposure (AE) Lock
Exposure lock can be used to recompose shots after metering a subject that will not be in the selected focus area in the final composition. Exposure lock is particularly effective when  or [ ] is selected for metering.
Use focus lock to lock focus on the current subject when AF‑A or AF‑C is selected for AF mode. When using focus lock, choose an AF‑area mode other than .
Locking Focus and Exposure
Position the subject in the selected focus point and press the shutter-release button halfway to set focus and exposure.
With the shutter-release button pressed halfway (q), press the A button (w).
Focus and exposure will lock while the A button is pressed. Exposure will not change even when the composition is altered.
An AE‑L icon will be displayed in the viewfinder or monitor.
Keeping the A button pressed, recompose the photograph.
Do not change the distance between the camera and the subject. If the distance to the subject changes, release the lock and focus again at the new distance.
Focus will lock while the shutter-release button is pressed halfway. Focus will also lock while the A button is pressed.
If  is selected for Custom Setting c1 [ ], exposure will lock while the shutter-release button is pressed halfway.
Focus will remain locked if you keep the A button pressed between shots, allowing several photographs in succession to be taken at the same focus setting.
Focus will also remain locked if you keep the shutter-release button pressed halfway between shots.