Canon MARK IV User Manual (en)

 Selecting the AF Point
Maximum f/stop: Up to f/2.8
With the 39 AF points indicated by  , high-precision, 
cross-type AF (both horizontal- and vertical-line 
sensitive) is possible during manual AF point 
selection. The cross-type sensors’ vertical-line 
sensitivity is approx. two times higher than their 
horizontal-line sensitivity. The remaining six AF 
points will be horizontal-line sensitive.
During automatic AF point selection, the number of 
cross-type AF points will decrease from 39 points to the 
19 points indicated with  . The remaining 26 AF points 
will be horizontal-line sensitive.
Maximum f/stop: f/4
High-precision, cross-type AF with the center AF point is possible. The 
remaining 44 AF points will be horizontal-line sensitive only.
Maximum f/stop: f/5.6 or f/8
With f/5.6 lenses, all the AF points will be horizontal-line sensitive only. 
With f/8 lenses, AF will be possible with the center AF point being 
horizontal-line sensitive only. AF will not work with the other AF points.
[Manual selection]
[Automatic selection]
In the case of zoom lenses whose maximum aperture varies depending 
on the lens focal length, the AF points will be horizontal-line sensitive only 
(They will not work as cross-type points). However, with the EF28-80mm 
f/2.8-4L USM lens, cross-type AF will work with the center AF point.
With the EF24mm f/2.8 or EF28mm f/2.8 lens, the three AF points on the 
extreme left and right of the Area AF frame will be horizontal-line 
sensitive (They will not work as cross-type points).
The following lenses and lens combinations have a maximum aperture of 
f/4, but cross-type focusing on 39 AF points is still possible as with lenses 
having an f/2.8 maximum aperture.
EF17-40mm f/4L USM, EF24-105mm f/4L IS USM,
EF70-200mm f/2.8L IS USM + Extender EF1.4X II, 
EF200mm f/2L IS USM + Extender EF2X II, 
EF300mm f/2.8L IS USM + Extender EF1.4X II, 
EF400mm f/2.8L IS USM + Extender EF1.4X II