Nikon D780 DSLR Camera (Body Only) - Manual (Page 129)

83 Camera Controls
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 [
Spot
metering ] or [ Center-weighted metering ] is selected for
metering.
Focus Lock
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 [ Auto-area AF ].
Locking Focus and Exposure
1
Position the subject in the selected focus point
and press the shutter-release button halfway to
set focus and exposure.