Canon EOS-1D Mark II Digial Instruction Manual page 154

C.Fn-20
1 or 2: Prevents the autofocus from being thrown off by any obstacle
passing between the camera and subject.
3 or 4: Effective when you want to consecutively photograph multiple
subjects located at random distances.
This Custom Function does not affect the AI Servo AF tracking speed.
C.Fn-21
1: Shooting is possible even while focus has not been achieved.
Even though the focus is not quite sharp, at least you will have a
picture.
C.Fn-00
0: For New Laser-matte screens.
1: For Laser-matte screens. Since the
EOS-1D Mark II's standard focusing screen is the Ec-CIII, C.Fn-
00-1 is already set upon factory shipment.
For C.Fn-00 when you change the focusing screen, see page 166.
154
AI Servo tracking sensitivity
AI Servo continuous shooting and
shooting priority
Focusing screen