Nikon D800 - Manual (Page 122)

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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 91), you can also focus on another subject at
the same distance and then use focus lock to recompose the
photograph. Focus lock is most effective when an option other
than auto-area AF is selected for AF-area mode (
0 93).
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.
2
Lock focus.
AF-C focus mode ( 0 91): With the
shutter-release button pressed
halfway (q), press the
A AE-L/
AF-L button (w) to lock both
focus and exposure (an AE-L
icon will be displayed in the
viewfinder). Focus will remain
locked while the
A AE-L/AF-L
button is pressed, even if you
later remove your finger from
the shutter-release button.
AF-S focus mode : Focus locks
automatically when the in-focus indicator appears, and
remain locked until you remove your finger from the shutter-
release button. Focus can also be locked by pressing the
A AE-
L/AF-L button (see above).
Shutter-release button
A AE-L/AF-L button