The information is best summarized in FAQ pages on Canon UK's website for the 7D, 40D, 50D, and 60D, but is also briefly summarized in a footnote on the Service and Support pages for the same models on the Canon USA website. Each of the cameras features a TTL-CT-SIR CMOS autofocus sensor whose AF points -- of which there are 19 in the EOS 7D, and nine in the other cameras -- are all cross-type points, sensitive to detail in both the horizontal and vertical axes when used with a maximum aperture of up to f/5.6.
However, eight historical models with maximum apertures brighter than f/5.6 at wide angle are nonetheless unable to use any of the focusing points as cross-type. Instead, when used with these lenses, the focus points will revert to detecting detail only in one or other axis, but not both simultaneously. The lenses in question are as follows:
The reasoning for this is said to be down to the combined optical characteristics of the AF sensor and these lens models. More details can be found in the Canon UK FAQ articles, or on the camera's support pages on the Canon USA website.
