Nikon D300 User Manual page 265

Focus tracking with lock-on allows you to select the length of
time that your camera will ignore an intruding object that
blocks your subject.
In other words, let's say you are focused on a bird flying past
you. As you pan the camera with the bird's movement, the
autofocus system tracks it and keeps it in good focus. As the
bird flies by, a road sign briefly interrupts the focus tracking
as the bird moves behind it and then re-emerges. How would
you feel if the bright, high-contrast road sign grabbed the
camera's attention and you lost tracking on the bird? That
would be quite aggravating, wouldn't it?
The D300(S) provides Focus tracking with lock-on to prevent
this from happening. The "lock-on" portion of this function
helps your camera keep its focus on your subject, even if
something briefly comes between the camera and subject. The
camera locks on to your subject doggedly if this function is
enabled.
Without Focus tracking with lock-on, any bright object that
gets between you and your subject may draw the camera's
attention and cause you to lose focus on the subject.
The camera provides a variable time-out period for the
lock-on functionality. Lock-on time-out allows an object that
stays between the camera and your subject for a
predetermined length of time to attract the camera's attention.
You can adjust the length of this time-out with a time period
from Short to Long.
Here are the screens and steps to configure Focus tracking
with lock-on (see Figure 4-9):
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