Quote Originally Posted by neuroanatomist
One thing to keep in mind - your camera body requires least f/5.6 for the autofocus system to work (the 1-series Canon bodies can AF with f/8 lenses). So with the Sigma18-200mm f/3.5-6.3, above 135mm when the max aperture drops to f/6.0, you'll lose the ability to autofocus. That's one reason the maximum aperture on the somewhat more expensive Canon 18-200mm is f/5.6 - so AF will work on all Canon bodies. Also, since the Sigma 18-200mm lens doesn't have full-time manual focusing, you'll need to remember to flip the switch from AF to MF when you zoom past 135mm (then back again if you want AF when you zoom out). All in all, I don't think it's worth the hassle - sort of defeats the purpose of the 18-200mm range convenience, IMO.
That is a common misunderstanding. The f3.5-6.3 zooms I've used focused fine along their entire range. I'm not sure why though--maybe they are chipped to tell the camera f/5.6?


If you want a 18-200mm lens, Denise, I'd recommend the Canon version.