Nikon D3X - Manual (Page 106)

80
N
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. It can also be used when the
autofocus system is unable to focus (pg. 82).
1
Focus.
Position the subject in the selected focus
point and press the shutter-release
button halfway to initiate focus.
2
Check that the in-focus indicator
(
I) appears in the viewfinder.
Single-servo AF
Focus will lock 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 AE-L/AF-L
button (see following page).