Hi all, I've personally tried only one USM lens, the canon 70-200 f/4 L one. you know focus is not only QUITE (you don't hear nothing other than a "zzz" - and sometimes only a "z") it's also faaaaast.
but this is not the best thing, as for many photographer the focus speed w/o USM is just bearable. the best thing, IMO, is the Full Time Manual Focus!! (don't know how the pentax AF lens work, with the Tamron one I have (17-34 2.8-4) I must switch the AF off every time I put it back in the bag, otherwise I risk to break the AF system by accidentally turning the focus ring doing it) by, Danilo.
