[matthewchj]

Originally Posted by RexK_Cozumel

While I do not own a cannon I would think that if the AF works for the lens it would not matter what the aperature is set to. I think that (someone correct me if I am wrong) the reason that AF does not work is that some cameras do not have a motor in the body so it will only autofocus if the lens has a motor. but I have never heard of a lens that will focus with one setting but not another.

Canon consumer and prosumer DSLRs require a lens that can open up to at least f/5.6 in order for AF to work. This is not due to the lens motor; all Canon lenses have motors built into them, but due to the sensitivity of the AF sensor which is built into the body.

This does not mean that you can't take a picture using AF at an aperture of say f/11. The lens just needs to be opened up to at least f/5.6, not set to f/5.6, as AF takes place when the lens is wide open.

Canon Professional cameras (1- series bodies) have a more sensitive AF sensor, and can AF with lenses f/8 or faster.