Nikon D6 Reference Manual page 223

2
Lock focus.
AF-C selected for autofocus mode:
Keeping the shutter-release button
q
pressed halfway ( ), press the center
w
of the sub-selector ( ) to lock focus.
Focus and exposure will remain
locked while the center of the sub-
selector is pressed, even if you
remove your finger from the shutter-
release button.
An AE-L icon will be displayed in the
viewfinder while the center of the
sub-selector is pressed.
AF-S selected for autofocus mode:
Focus locks while the shutter-release button is pressed
halfway. You can also lock focus by pressing the center of
the sub-selector.
171
Choosing How the Camera Focuses (Focus)