Canon MARK II EOS 5D Instruction Manual page 83

Lens' Maximum Aperture and AF Sensitivity
With lenses whose maximum aperture is larger than f/5.6
With the center AF point, cross-type AF (sensitive to both vertical and
horizontal lines) is possible. The remaining eight AF points are
horizontal-line sensitive or vertical-line sensitive.
With lenses whose maximum aperture is larger than f/2.8
With the center AF point, high-precision, cross-type AF sensitive to both
vertical and horizontal lines is possible. The center AF point's sensitivity
to vertical and horizontal lines is about twice as sensitive as the other
AF points. The remaining eight AF points are horizontal-line sensitive or
vertical-line sensitive.
* Except with the EF 28-80mm f/2.8-4L USM and EF 50mm f/2.5 Compact Macro
lenses.
The Assist AF points used in the AI Servo AF
mode are within the spot metering circle and not
displayed like normal AF points. They function as
follows:
: Vertical-line sensitive with f/2.8 and f/5.6
lenses, as with the center AF point.
: Horizontal-line sensitive with f/5.6 lenses.
S Selecting the AF PointN
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