Nikon D780 DSLR Camera (Body Only) - Manual (Page 219)

173 Shooting Settings
Autoexposure Lock
Use autoexposure lock to recompose photographs after setting
exposure for a specific area of the subject. Exposure lock comes
in handy when the area used to set exposure is much brighter
or darker than its surroundings.
1
Select [Center-weighted metering] or [Spot
metering] (
0 166).
2
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
button to lock exposure (if you are
using autofocus, confirm that the
I in-focus indicator
appears in the viewfinder).
• While exposure lock is in effect, an
AE-L indicator will appear in the
viewfinder.