Nikon D300 User Manual page 89

[] The AF-ON Button
For the purpose of focusing the camera, pressing
the AF-ON button has the same effect as pressing
the shutter-release button halfway.
AF-ONbutton
[] 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 AF-ON button is pressed. This
allows the camera to track focus while attempting to predict where the
subject will be when the shutter is released.
[] See Also
For information on using focus priority in continuous-servo AF, see
Custom Setting al ([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).
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