Originally posted by darthku My first DSLR was the K10D and now I have a K20D. Recently, I bought a used 40D because I was interested in the 70-200 f/2.8 IS USM lens (BTW... that lens is AWESOME). I can honestly say Canon's AF system is just better than the Pentax system. When comparing SDM to USM, SDM takes longer as it kind of does a midpoint search for the correct AF. While the Canon swings to the correct AF without having to do any minor adjustments.
This may be true, but I think it's important to add that the difference isn't that significant. I have a K10D which I typically use with the DA 40mm Limited. I have a friend who is a Canon shooter, and I've borrowed his 30D with 24-70mm f/2.8 L and 70-200mm f/2.8 L lenses.
Now, of course the lenses aren't even apples to watermelons (and no question that's one of Pentax's fastest AF lenses), and granted that's a slightly-older Canon, but basically, the difference in focus speed isn't appreciably different, even in low light. (In fact, my K10D with that lens is significantly more capable in low light than his previous EOS 10D with the Canon f/1.4 50mm.)
None of this is measured, but I've used both enough to be sure of my assertion. If you can measure a difference, it's not enough to matter. The important exception is that the Pentax AF-C mode can't track a kid running towards me to save its life, while the Canon can.
(Those Canon zoom lenses
are gorgeous, by the way. Even if I do prefer primes for my own use.)