Why does Canon limit or disable autofocus below certain f-numbers?

Question

One thing that I've noticed with Canon DSLR camera is they will disable or limit autofocus when the lens is below certain aperture values:

• On Rebels since the XTi (400D) as well as the 30D, the center point is high-precision cross-type only when the aperture is at least f/2.8. If below f/2.8 but at least f/5.6, all points function as horizontal sensitive only. Below f/5.6, autofocus does not function.
• On the 40D, 50D, 60D, and 7D, the center point is high-precision twin cross-type only when the aperture is at least f/2.8. If below f/2.8 but at least f/5.6, all points function as cross type, but the center point is single cross-type and normal precision only. Below f/5.6, autofocus does not function.
• On the Canon EOS 5D Mark II, the center point is high-precision cross-type only when the aperture is at least f/2.8; the other points are either horizontal or vertical sensitive (but not both). If below f/2.8 but at least f/5.6, the center point remains cross-type but is normal precision only; all other points continue to function normally. Two of the six invisible Assist AF points used in AI Servo AF are cross-type at f/2.8, with the rest horizontal sensitive; if below f/2.8 but at least f/5.6, they are all horizontal sensitive. Below f/5.6, autofocus does not function.
• On the Canon EOS-1D Mark IV, the following applies:
  • If at least f/2.8, all 45 autofocus points will function, with 39 points high-precision cross-type. (For the Canon EOS-1D Mark III and 1Ds Mark III, 19 of the 45 points are high-precision cross-type.)
  • If at least f/4, all 45 autofocus points will function, but only the center point is high-precision cross-type.
  • If at least f/5.6, all 45 autofocus points will function as horizontal sensitive only.
  • If at least f/8, only the center point functions as horizontal sensitive only. The other points will not work.
  • Below f/8, autofocus does not function.
  • There are exceptions:
    • Certain lenses do not allow the far left and right points to act as cross-type points (only horizontal sensitive) even though they have a maximum aperture of f/2.8.
    • Some lenses and lens/teleconverter combinations allow cross-type functionality with all 39 of the 45 points (such as the EF 24-105mm f/4L lens; this applies to the 1D Mark IV only).

Nikon, Olympus, and Pentax do not have these limitations, even if autofocus may be slow or unreliable beyond f/5.6. (Sony/Minolta requires at least f/6.3, with one exception for a 500mm f/8 autofocus mirror lens, and the Alpha 850 and 900 have a center AF point with high precision at f/2.8.) For example, on Pentax, 9 of the 11 points are cross-type and remain so at all apertures. The Pentax AF system has been reported to work at f/8 and beyond, though extremely slowly. Why does Canon choose to limit AF functionality instead of attempting to focus with reduced performance beyond the above cutoffs?

Answer

The auto focus systems are not capable of using the highest precision cross type focusing points if you do not use a large aperture lens. They aren't artificially limiting the precision, they are simply working within the constraints of the maximum aperture.

Canon puts these limits in to ensure reliable AF. If you don't believe that theory, add a piece of tape between your teleconverter and lens, on the top three left pins. This will trick your body into thinking that you don't have a teleconverter attached, and your lens is simply a 300/4.0 or whatever you have.

Furthermore, only the Canon 1-series cameras are capable of AF with a f/8 lens, albeit at the center point only. Usually this setup comes from combining a long lens with a extender.

This really comes down to a issue of user experience. Sure, they could include ISO 204800 or 409600, but the quality would not make anyone happy. They are trying to keep the majority of users satisfied with what the camera can do, and most consumers are satisfied if AF doesn't hunt endlessly.

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