Canon MARK IV User Manual (en)

 Selecting the AF Point
To select a horizontal AF point, turn 
the <6> dial.
To select a vertical AF point, turn the 
<5> dial.
If all the peripheral AF points light up, 
automatic AF point selection will take 
With the EOS-1D Mark IV, higher precision AF is possible with a 
maximum aperture brighter than f/2.8 on a lens or when combined with 
an Extender.
Selecting with the Dial
Lens’ Maximum Aperture and AF Sensitivity
With [8C.Fn III -10: Selectable AF point], the selectable AF points can 
be limited to 19 or 11 AF points or to the inner or outer 9 AF points 
With [8C.Fn III -8: AF expansion w/selected pt], you can change the 
AF expansion to the left/right AF points, surrounding AF points, or all 45 
AF points (p.222).
If you use a super telephoto lens equipped with an Image Stabilizer and 
AF stop button, when [8C.Fn III -6: Lens AF stop button function] is 
set to [7: Spot AF], spot AF will be possible (p.220).
By registering the AF point you use frequently, you can switch to it 
instantly by setting C.Fn III -11-1/2 (p.224) or C.Fn III -6-6 (p.220).
If focus cannot be achieved with the external, EOS-dedicated Speedlite’s 
AF-assist beam, select the center AF point.