Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 99

AF Operation and Maximum Lens Apertures
With maximum apertures up to f/5.6:
Cross-type focusing (vertical and horizontal lines detected
simultaneously) is possible with all AF points. With certain lenses (see
below), the off-center AF points will detect only vertical or horizontal
lines (no cross-type focusing).
With maximum apertures up to f/2.8:*
High-precision cross-type focusing (vertical and horizontal lines
detected simultaneously) is possible with the center AF point. The
center AF point is about twice as sensitive as the other AF points to
detect vertical and horizontal lines.
The remaining eight AF points enable cross-type focusing in the same
way as when using lenses with maximum apertures up to f/5.6.
* Except with the EF28-80mm f/2.8-4L USM and EF50mm f/2.5 Compact Macro.
If you use an Extender (sold separately) and the maximum aperture
becomes smaller than f/5.6, AF shooting will not be possible (except in
[u+Tracking], [FlexiZone - Multi], and [FlexiZone - Single] during Live
View shooting). For details, refer to the Extender's instruction manual.
Lenses not Enabling Cross-Type Focusing with all AF Points
The following lenses enable only horizontal-line sensitive focusing with
the three AF points on the left and the three AF points on the right and
only vertical-line sensitive focusing with the top and bottom AF points.
Cross-type focusing is possible only with the center AF point.
EF35-80mm f/4-5.6
EF35-80mm f/4-5.6 II
EF35-80mm f/4-5.6 III
EF35-80mm f/4-5.6 USM
EF35-105mm f/4.5-5.6
EF35-105mm f/4.5-5.6 USM
EF80-200mm f/4.5-5.6 II
EF80-200mm f/4.5-5.6 USM
S Selecting the AF PointN
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