yea I have to agree, I'm not sure why they do that, but I suspect it has something to do with lens aberration and other such things that would make the auto focus not work properly at the outer edges if you were using a lens that bent the edges or something like that as an example.

The 9 focus points go all the way back to the 10D, it's an artifact of a prosumer technology; no firmware upgrade will ever change this. It has nothing to do with the lenses, in terms of the path light travels through the lens. The Canon EOS autofocus system points are not field-upgradable, it's literally fixed in silicon mechanically mounted in the camera. Certain aspects of the how the available points are used can be changed via firmware updates, but not the number of points and their location in the frame.

The Canon EOS autofocus system operates with a completely separate image sensor that uses light from the partially silvered mirror in the DSLR that provides the viewfinder image. On later cameras, there are some "hidden" focus assist points and the autofocus image sensor is upgraded to have higher sensitivity. Still, the autofocus control is handled by these 9 points in this separate imaging sensor, and they're in a diamond shape in the center of the frame.

The lens aperture affects performance, because that sets the maximum amount of light available via the half-silvered mirror. For a given camera though, all Canon lenses produce exactly the same output image circle to the photographic image sensor; the prosumer cameras like the 50D use a smaller circle than the full-frame cameras like the 5D series. But other than aperture, the type of lens is immaterial to light impinging on the autofocus image sensor, because the same circle is covered no matter the lens type.

In contrast, the professional 1D series EOS cameras have a 45-point focus system that covers a wider area of the frame, arranged in an ellipse. It takes a LOT more computational processing power, and also a higher performance AF image sensor, to use this system. That situation in turn requires more electrical power, which is inimical to the smaller form factor of the prosumer package. Additionally, the AF system in the 1D series operates a lot faster than prosumer cameras to obtain focus-lock, and to hold it for the AI focus mode, and this means running the DIGIC image processor hardware at a faster clock rate, again using more power that would not be so available in a prosumer camera form-factor.

This small number of points was one of the reasons I really had to stop using my 20D ... photographing belly dance troupes was really hard, because numerous belly dancers would fit in-between the points on the diamond. Even moving solo dancers were a problem because their fast-moving body shape is not reflected in the diamond pattern. Legs and the head are not served well but the bottom and top of the diamond, and even a belly dancer filling the frame could easily be missed since the middle parts of the diamond scarcely covered the dancer. Using the 20D required select autofocus (SA) by lining up a dot on the 9-point array with the dancer, constantly moving the select point around using the multicontroller to dynamically recompose in real time for the moving dancer ... ugh-o-rama that was taxing.

Despite the attractive price points of cameras like the Rebel TSi, or the 50D, and with indifference to the issue of build quality, the whole entire reason I needed to go with the professional 1D series cameras is because the 45-point ellipse is enormously stronger and much faster. This system much more effectively analyzes the scene, and for cameras like the 1D Mark III, it's wicked fast. In dim light situations, much like the 20D, it's often faster still to use SA, but with a 45-point ellipse there's much greater coverage in the viewfinder scene.

One could say that the 5D Mark II represents a middle-range EOS camera. It has the imaging quality of the higher-cost professional EOS models, but other aspects like the build quality, and the AF sub-system have their roots strictly in Canon's prosumer EOS family.

Blessings and Light,
M