Nikon D3 User Manual page 101

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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.
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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], pg. 304). For information on
using release priority in single-servo AF, see Custom Setting a2 ([AF-S
priority selection], pg. 305). For information on preventing the camera
from focusing when the shutter-release button is pressed halfway, see
Custom Setting a5 ([AF activation], pg. 308).
B button
B button for
vertical shooting
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