AF Sensitivity and Lens’ Maximum Aperture
The EOS-1D Mark II’s AF points are all horizontal-line sensitive. However, bright lenses with a large maximum aperture enable certain AF points to work as cross-type sensors for higher AF precision.
(1) With lenses whose maximum aperture is f/2.8 or larger, 
the AF points highlighted in the diagram will work as cross-type sensors sensitive to both vertical and horizontal lines. The remaining 38 AF points will only be horizontal-line sensitive. The cross-type sensor’s vertical-line sensitivity is about three times higher than its horizontal-line sensitivity.
(2) With the following L-series lenses whose maximum aperture is f/4 or brighter, the center AF point will be a cross-type sensor with or without an Extender attached. The remaining 44 AF points will only be horizontal-line sensitive (except with EF 70-200mm f/2.8L USM + Extender EF 1.4x).
·EF 28-80mm f/2.8-4L USM
·EF 300mm f/4L USM
·EF 300mm f/4L IS USM
·EF 400mm f/4 DO IS USM
·EF 500mm f/4L IS USM
·EF 600mm f/4L USM
·EF 600mm f/4L IS USM
·EF 70-200mm f/4 L USM
¡With Extender EF 1.4x or EF 1.4x II:
·EF 200mm f/2.8L USM
·EF 200mm f/2.8L II USM
·EF 300mm f/2.8L USM
·EF 300mm f/2.8L IS USM
·EF 400mm f/2.8L USM
·EF 400mm f/2.8L II USM
·EF 400mm f/2.8L IS USM
·EF 70-200mm f/2.8L IS USM
¡With Extender EF 2x or EF 2x II:
·EF 135mm f/2L USM
·EF 200mm f/1.8L USM