Nikon D300 Complete Manual page 378

V1.02
unanswered questions, especially for those used to using one
of the previous autofocus systems.
Autofocus is achieved using fifty-one small contrast sensors.
The approximate positions of these sensors appear in the
viewfinder in some situations; normally, the viewfinder only
shows the area in which the autofocus sensors live:
With autofocus active, one or more sensors may light up,
indicating the sensor(s) that will be used for focus:
The actual part that contains the autofocus sensors—called
the CAM3500DX—looks like this:
Thom Hogan's Complete Guide to the Nikon D300 Page 378