Canon EOS Cameras: Focus Point Aperture Limits

Values indicate the maximum aperture at least neccessary to trigger the corresponding autofocus points (i.e. if your lens - teleconverter - combo results in F8 and your camera requires F5.6 at least, there will be no autofocus).

Lin = linear type sensors

CX = center cross type sensors

PX = peripheral cross type sensors

In most if not all cases F5.6 sensors will do the main job of focusing. If you use a fast lens (F2.8 and faster), additional highly sensitive sensors will add some accuracy. Linear sensors are less depending on lens speed, but only capture horizontal details and work slower than cross sensors. Some camera models' cross sensors will turn into linear sensors when working with slower lenses.

EOS 400D: With slower lenses the F2.8 center cross turns into a F5.6 linear type sensor (see camera's White Paper p.10).
EOS 1D Mark IV: The F4 center cross will turn into a F8 linear, the outer F2.8 cross will turn into F5.6 linear sensors (see White Paper pp.13 resp. p17). Same is valid for the 1D Mark III & 1Ds Mark II. The EOS 1Ds Mark III's White Paper is not clear in this point.

Generally speaking, information about this feature is hard to find and sometimes differs adventurously from source to source. Adding to the confusion in some cases, firmware updates increasing those sensitivities were released long after the cameras initially did hit the market.

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