Canon EOS 1D Instruction Manual page 85

Lens' Maximum Aperture and AF Sensitivity
The EOS-1D Mark III can execute high-precision AF with lenses whose
maximum aperture is f/2.8 or larger.
With f/2.8 and faster lenses*
Assist AF points
Cross-type points
With lenses whose maximum aperture is larger than f/4
If the maximum aperture of the lens or Extender and lens combination
is faster than f/4, the center AF point will work as a high-precision,
cross-type point sensitive to both horizontal and vertical lines. The
remaining 18 AF points and the 26 Assist AF points work as horizontal-
line sensitive AF points.
With lenses whose maximum aperture is larger than f/5.6
With lenses whose maximum aperture is larger than f/5.6, all the AF
points, including the Assist AF points, will be horizontal-line sensitive only.
With lenses whose maximum aperture is larger than f/8
With lenses whose maximum aperture is larger than f/8, AF will be
possible with the center AF point which will be horizontal-line sensitive.
AF will not work with the other AF points.
When the EF70-200mm f/2.8L USM is used with an Extender, use the center
AF point only. The other AF points may cause a focusing error.
When using the EF24mm f/2.8 or EF28mm f/2.8 lens,
the 13 AF points shown on the right can be used as
cross-type points. The remaining 6 AF points will be
horizontal-line sensitive.
With the 19 AF points indicated by ,
high-precision, cross-type AF (both
horizontal- and vertical-line sensitive) is
possible. With cross-type AF, vertical-line
detection is about 2 times as sensitive as
horizontal-line detection. The remaining
26 Assist AF points are horizontal-line
sensitive only.
* Excluding the EF24mm f/2.8 and EF28mm f/2.8.
S Selecting the AF Point
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