Many experienced Canon EOS shooters have long requested the ability to autofocus with lenses slower than the traditional f/5.6 limit — especially nature, wildlife and sports photographers. Any shooter who routinely relies on long telephoto lenses, and who wants to expand their reach with a Canon-brand tele extender, can find themselves wanting the same capability.
Typical real-world examples might be attaching a 2x extender to an f/4 lens, like an EF 500mm f/4L IS II. Or, attaching a 1.4x extender to an f/5.6 lens, such as an EF 400mm f/5.6L. Either way, the lens + extender’s widest effective aperture would equate to f/8.
Canon first answered these requests for f/8 AF compatibility with the EOS 3 (a 35mm film camera), introduced in 1998, which permitted AF with lenses from f/5.6~f/8 at the center AF point only. Subsequent efforts, until now, have likewise involved AF at f/8 only at the center AF point, or in some cases allowing four surrounding AF points to provide AF Point Expansion around that center AF point.