Hi I've just gotten hold of a EF 20mm f2.8mm with USM autofocus. Its has a a strange problem that I've never come across before. When focussing in the viewfinder it never hits focus, just hunts from shortest range to infinity and back. When focussing using live view it works fine. Manual also works fine. I've tried this on both my 7D and 5D mkii and it exhibits the same behaviour. Anybody else had this and how was it resolved, if at all? I can manage in manual mode - it being a wide angle focus is not so critical unless you're doing close up at wide apertures. thanks in advance for any help!
Live view will help give consistent results over the viewfinder. Since I don't have the lens its hard to say from experience if its a lens issue or not, how much testing have you done? The viewfinder is going to need the focus point you are using to hit something with decent contrast for focus to grab onto, so depending on the subject/distance/and lighting i could see where it may struggle. A wide angle is going to be more susceptible in less than optimal conditions, and if your using all focus points it may be worse as its trying to figure out which one(s) it should use.
Hi It's definitely the lens. I've tested it extensively in bright sunshine with high contrast targets, used all of the available autofocus points in each camera individually and in full auto. Nothing works, just hunts backwards and forwards as described. Focusing through the viewfinder applies phase detection. Live View applies contrast detection.
Well, I didn't pay much for it so it's not the end of the world that it doesn't work completely. I was just hoping that someone had had a similar experience and could say it was x or y. Then it would help me decide whether it was worth sending for an evaluation. Thanks anyway.
I would definitely send it in for evaluation. Did 3rd buy it new or used? Just looked on Canon site and new that lens is over$500. I would have it checked out.
