Nikon D300 User Manual page 726

AF point of the pattern, even though you have
points, or 51 points
Viewfinder provides the primary autofocus; however, the
surrounding sensors (in the pattern you've selected in
setting a3) are also active. If the subject moves and the
primary AF point loses its focus, one of the surrounding
sensors will grab the focus quickly.
If the subject is moving slowly or predictably, you can use a
smaller pattern, such as the
setting a3. If the subject's movement is more erratic or
unpredictable, you might want to increase the number of AF
points involved. If
for subjects that are very unpredictable and move quickly.
Capturing a Bird in Flight
Let's imagine that you are photographing a bird perched in
a tree but you want some shots of it in flight. You are
patiently waiting for it to fly. Your camera is set to
Dynamic-area AF with all 51 points active so that
autofocus never fully locks and will track the bird instantly
when it starts flying. You've already established focus
with the AF point you selected using the Multi Selector,
and you are holding the Shutter-release button halfway
down to maintain focus. You've also set the Release mode
to CH (Continuous high) so that you can fire off rapid
bursts of images (6-8 per second). Suddenly, and faster
than you can react, the bird takes to flight. By the time you
can get the camera moving, the bird has moved to the left
in the Viewfinder and the focus tracking system has
reacted by instantly switching away from the primary AF
point you established focus with and is now using other
selected. The AF point you can see in the
9 points
9 won't do it, try 21, and finally even
726
9 points,
Custom
selection in Custom
21
51