Nikon Df User Manual page 80

Autofocus mode can be selected by pressing the AF-mode button
and rotating the main command dial until the desired setting is
displayed in the viewfinder.
A
The B Button
For the purpose of focusing the camera, pressing
N
the B button has the same effect as pressing
the shutter-release button halfway.
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 the B button is pressed. This
allows the camera to track focus while attempting to predict where the
subject will be when the shutter is released.
A
See Also
For information on using focus priority in continuous-servo AF, see
Custom Setting a1 (AF-C priority selection, 0 246). For information
on using release priority in single-servo AF, see Custom Setting a2 (AF-S
priority selection, 0 246). See page 175 for information on the
autofocus options available in live view.
60
AF-mode button
AF-S
Main command dial
AF-C
B button