Nikon D300s - Manual (Page 101)

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A The B Button
For the purpose of focusing the camera,
pressing the
B button has the same effect as
pressing the shutter-release button halfway;
note, however, that vibration reduction
(available with VR lenses) can only be engaged
by pressing the shutter-release button halfway.
A Predictive Focus Tracking
In continuous-servo AF, 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, 0 263).
For information on
using release priority in single-servo AF, see Custom Setting a2 (
AF-S
priority selection, 0 264).
For information on preventing the camera
from focusing when the shutter-release button is pressed halfway, see
Custom Setting a5 (
AF activation, 0 266).
B button