Autoexposure Lock - Nikon D3400 Reference Manual

Autoexposure Lock

Use autoexposure lock to recompose photographs after using
M (Center-weighted metering) and N (Spot metering) to
meter exposure; note that autoexposure lock is not available in
i or j mode.
1
Lock exposure.
Position the subject in the selected
focus point and press the shutter-
release button halfway. With the
shutter-release button pressed
halfway and the subject positioned in
the focus point, press the A (L)
button to lock exposure.
While exposure lock is in effect, an AE-L
indicator will appear in the viewfinder
and the monitor.
Shutter-release button
A (L) button
P, S, A, and M Modes
113