Nikon D300 User Manual page 719

the shutter fires a few milliseconds later. In other words, if the
subject is
moving toward you, the lens focuses slightly in front of your
subject so that the camera has time to move the mirror up and
get the shutter blades out of the way. It takes 45 milliseconds
for the camera to respond to a press of the Shutter-release
button.
Let's talk about the practical use of these functions. If you are
shooting an air show, for instance, in 45 milliseconds a
fast-moving airplane can move enough to slightly change the
focus area by the time the shutter opens. If you press the
Shutter-release button in one smooth motion all the way to
shutter release, first autofocus occurs and then the mirror
moves up and the shutter starts opening. That takes about 45
milliseconds in the D300(S). In the time it takes for the
camera to respond to your press of the Shutter-release button,
the airplane has moved slightly, which just barely throws the
autofocus off. With predictive focus tracking, the camera
predicts where the airplane will be when the image is actually
exposed and adjusts the focus accordingly.
Predictive Focus Tracking
Let's say you were playing a ball game and you threw the ball
to a running player. You would have to throw the ball slightly
in front of the receiving player so that he and the ball arrive in
the same place at the same time. Predictive focus tracking
does something similar for you. It saves you from trying to
focus your camera in front of your subject and waiting 45
milliseconds for it to arrive. That would be a bit hard to time!
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