Nikon D800 - Manual (Page 306)

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When AF-S ( 0 91) is selected for viewfinder photography, this
option controls whether photographs can be taken only when the
camera is in focus (focus priority) or whenever the shutter-release
button is pressed (release priority) in single-servo AF.
Regardless of the option selected, if the in-focus indicator ( I ) is
displayed when AF-S is selected for autofocus mode, focus will lock
while the shutter-release button is pressed halfway. Focus lock
continues until the shutter is released.
a2: AF-S Priority Selection
G button ➜ A Custom Settings menu
Option Description
G Release
Photos can be taken whenever the shutter-release
button is pressed.
F Focus
Photos can only be taken when the in-focus indicator
(
I) is displayed.