71Using Autofocus (AF)3AF Sensitivity and the Lens’ Maximum ApertureThe EOS-1Ds’s AF points are all horizontal-line sensitive. However, bright lenseswith a large maximum aperture enable certain AF points to work as cross-typesensors for higher AF precision.(1) With lenses whose maximum aperture is f/2.8 or faster(lower f/number), the AF points highlighted in the illustrationwill be cross-type sensors sensitive to both vertical andhorizontal lines. The remaining 38 AF points will only behorizontal-line sensitive. The cross-type sensor’s vertical-line sensitivity is three times higher than the horizontal-linesensitivity.(2) With the following L-series lenses whose maximumaperture is brighter than f/4 by lens only or with Extenders,the center AF point will be a cross-type sensor for high-precision AF. The remaining 44 focusing points will only behorizontal-line sensitive (except with EF 70-200mm f/2.8LUSM + Extender EF 1.4x).EF 28-80mm f/2.8-4L USMEF 300mm f/4L USMEF 300mm f/4L IS USMEF 400mm f/4 DO IS USMEF 500mm f/4L IS USMEF 600mm f/4L USMEF 600mm f/4L IS USMEF 70-200mm f/4L USM• With Extender EF 1.4x or EF 1.4x II:EF 200mm f/2.8L USMEF 200mm f/2.8L II USMEF 300mm f/2.8L USMEF 300mm f/2.8L IS USMEF 400mm f/2.8L USMEF 400mm f/2.8L II USMEF 400mm f/2.8L IS USMEF 70-200mm f/2.8L IS USM• With Extender EF 2x or EF 2x II:EF 135mm f/2L USMEF 200mm f/1.8L USM
