Nikon D3 - Manual (Page 10)

10
Focus tracking with lock-on controls how the camera re-
sponds to abrupt changes in focus distance. It can be used
to prevent the camera from immediately refocusing when
objects move through the space between the subject
and the camera, ensuring that the camera will continue to
track the main subject once the obstacle has moved away.
Choose the length of time before the camera refocuses
from Long, Normal, and Short.
Camera will wait before
refocusing on objects
behind the subject
Camera will wait before
refocusing on objects
behind the subject
Camera will track and
focus on objects the
same distance from the
camera as the subject
Focus Tracking with Lock-On
Autofocus