Canon EOS 1D Mark II N Instruction Manual page 169

C.Fn-20
0: Standard
1: Slow / 2: Moderately slow
Prevents the autofocus from being thrown off by any obstacle passing
between the camera and subject.
3: Moderately fast / 4: Fast
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
0: Disable
1: Enable
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: k Ec-N,R
For New Laser-matte screens.
1: l Ec-A,B,C,Cll, Clll, D, H, I, L
For Laser-matte screens.
2: p Ec-S
For Super precision matte screens.
EOS-1D Mark II N'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 180.
AI SERVO tracking sensitivity
Drive speed priority AI SERVO
Focusing Screen
3 Custom Function Settings
169