Nikon D810 - Manual (Page 120)

96
Focus Lock
Focus lock can be used to change the composition after
focusing, making it possible to focus on a subject that will not be
in a focus point in the final composition. If the camera is unable
to focus using autofocus (0 87), focus lock can also be used to
recompose the photograph after focusing on another object at
the same distance as your original subject. Focus lock is most
effective when an option other than auto-area AF is selected for
AF-area mode (0 90).
1
Focus.
Position the subject in the
selected focus point and
press the shutter-release
button halfway to initiate
focus. Check that the in-
focus indicator (I) appears
in the viewfinder.