Nikon D300 User Manual page 717

Single-servo AF Mode (S)
Single-servo AF works best when you select the proper mode
for your subject. Is the subject moving or standing still?
Subject is not moving: When you press the Shutter-release
button halfway down, the AF Module quickly locks focus on
your subject and waits for you to fire the shutter. If your
subject starts moving and you don't release pressure on the
Shutter-release button to refocus, the focus will be obsolete
and useless. Once you have focus lock, take the picture
quickly. This mode is perfect for non moving subjects or in
some cases even very slowly moving subjects.
Subject is regularly moving: This will require a little more
work on your part. Since the AF system locks focus on your
subject, if the subject moves even slightly, the focus may no
longer be good. You'll have to lift your finger off of the
Shutter-release button and reapply pressure halfway down to
refocus. If the subject continues moving, you'll need to
continue releasing and pressing the Shutter-release button
halfway down over and over to keep the focus accurate. If
your subject never stops moving, is moving erratically, or
stops only briefly, Single-servo AF (S) is probably not the
best mode to use. In this case, Continuous-servo AF (C) is
better because it never locks focus, so you are able to follow
your subject's movement.
717