fensterbme posted a reply:
More light makes auto focus easier/quicker regardless. Autofocus is done before the aperture is stopped down with the largest opening possible to gather the most light.
On the 20D, 30D and the newer Canon DSLR's (including the XTi/400D) when a lens is used with a max aperture of f/2.8 or greater a secondary focusing array that results in three times faster AF over a lens that doesn't do f/2.8. I believe on the new 40D that secondary grid is active even down to f/5.6 which should help speed up AF when slower consumer grade lenses are used.
@h*lee: I would tend to follow Steve and say try using AI Servo AF mode, if you weren't that would be your problem... But 'yes' without a doubt a faster lens will result in faster autofocusing.
@jzackojones: You would be wrong...
@jameshamiltonphotography: Regardless of the seleted f/stop AF happens before the aperture blades stop down the lens (if it did you would see your camera get very dark in the viewfinder much like if you use the DoF preview button), so if set your aperture to f/11 it's not going to take any longer to focus.
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