Nikon Camera F3AF - Manual (Page 33)

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Focus lock
The AF-Nikkor lens has two focus lock buttons ® on the lens
barrel, one on the side and the other on the A-M switch. You can
use either of these buttons, depending on your choice of hori­
zontal- or vertical-format shooting. During autofocus operation,
depressing the focus lock button allows you to lock in the auto­
focus distance setting on the lens. Autofocus begins once again
as soon as you release the button.
To take pictures with the main subject off-center, first center the
main subject within the focusing frame in the viewfinder, depress
the shutter release button halfway and make sure both focus
indicators light up: then depress the focus lock button and, while
holding it in, recompose and shoot.