Nikon D4S - Manual (Page 121)

99
N
A
The B Buttons
For the purpose of focusing the camera, pressing
either of the
B buttons has the same effect as
pressing the shutter-release button halfway
(note that the
B button for vertical shooting
can only be used when the vertical shooting
shutter-release button lock is unlocked;
0 42).
A
Predictive Focus Tracking
In AF-C mode, the camera will initiate predictive
focus tracking if the subject moves toward or
away from the camera while the shutter-release
button is pressed halfway or either of the
B
buttons is pressed. This allows the camera to
track focus while attempting to predict where
the subject will be when the shutter is released.
B button
B button for
vertical shooting