Nikon D3S User Manual page 117

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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, however, that vibration
reduction (available with VR lenses) can only
be engaged by pressing the shutter-release
button halfway.
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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 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.
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See Also
For information on using focus priority in continuous-servo AF, see
Custom Setting a1 (AF-C priority selection, 0 295). For information on
using release priority in single-servo AF, see Custom Setting a2 (AF-S
priority selection, 0 296). For information on preventing the camera
from focusing when the shutter-release button is pressed halfway, see
Custom Setting a5 (AF activation, 0 298).
B button
B button for
vertical shooting
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