Nikon D300 - Manual (Page 89)

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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.
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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 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.
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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. 267).
For information on
using release priority in single-servo AF, see Custom Setting a2 ([AF-S
priority selection], pg. 268).
For information on preventing the camera
from focusing when the shutter-release button is pressed halfway, see
Custom Setting a5 ([AF activation], pg. 271).
B button