Nikon D6 Reference Manual page 856

a: Autofocus
a1: AF-C Priority Selection
Choose whether photos can be taken before the camera
focuses when AF-C is selected.
Option
G [Release]
[Focus +
B
release]
[Release +
E
focus]
F [Focus]
Regardless of the option chosen for [AF-C priority selection],
focus will not lock when AF-C is chosen for autofocus mode;
instead, the camera will continue to adjust focus until the
shutter is released.
804 A Custom Settings: Fine-Tuning Camera Settings
G button U A Custom Settings menu
Photos can be taken whenever the shutter-release
button is pressed (release priority).
Photos can be taken even when the camera is not in
focus, but if the subject is dark or low contrast and
the camera is in continuous-release mode, priority
will be given to focus for the first shot in each series.
For the remaining shots, priority will be given to
frame rate regardless of focus. This helps ensure that
the first shot in each series is in focus.
Photos can be taken even when the camera is not in
focus, but if the subject is dark or low contrast and
the camera is in continuous-release mode, the frame
advance rate will slow for improved focus. Focus is
prioritized over frame rate.
Photos can be taken only when the camera is in
focus (focus priority).
Description