Nikon D3s Technical Manual page 10

Autofocus
Focus Tracking with Lock-On
By preventing the camera from refocusing when your
subject is briefl y obscured by another object, focus
tracking with lock-on ensures that the camera will con-
tinue to track the main subject once the obstacle has
moved away (focus mode C only). Choose the length
of time before the camera refocuses from 5 (Long), 4, 3
(Normal), 2, and 1 (Short). To refocus before lock-on
ends, press the shutter-release button halfway or press
the AF-ON button again.
6
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
Camera will wait before
refocusing on objects in
front of the subject