EF-S 17-85mm IS USM Lens AF issues with 600D

Pitsy

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Hi,

I've recently purchased a 600D and decided (based on reviews etc.,) that it would be a good idea to invest in an EF-S 17-85mm f/4-5.6 IS USM lens as an all round lens and keep the standard 18-55mm lens as back-up, mainly, as I imagine, the 17-85 will give me better quality photos but also to have better zoom range. But I have found that in Auto mode, the AF will not work when using the viewfinder, but works fine when I switch to live-view. When I try other modes, the AF will only work in AI Servo mode when using the viewfinder, but is fine in one-shot mode when using live-view.

Has anyone else experienced this? Does anyone have any suggestions?
 
I have this same lens on my 40D. I've never experienced the issues you describe. It does tend to hunt around a lot in low light, but that can be expected given its max. aperture range.

Can you be more specific about the lens behavior and the scenarios in which it doesn't work?
 
I thought it may be down to light so I took the camera and lens outside at the weekend and it still behaved in the same way. When using the viewfinder in auto mode, you could hear the focus motor turning in the lens then stop, but will not get a focus lock and the green light indicating a focus lock would flash, even when I then manually focus using the ring but still in AF mode. Hit the button to switch to liveview mode and it will focus no problem. This is all in Auto mode.

If I change the setting to program or manual or any other mode, I have to change the AF mode from one-shot to AI Servo for me to us the viewfinder with the auto focus. As before though, one-shot AF mode will work in live-view mode. Manual focus works fine. Just to confuse matters more, the 18-55mm lens that came with the camera works fine.

I have noticed a small speck on the inside of the lens (which is annoying as I can't clean it off), which I am unsure about how it got there as I have been assure that the lens has been hardly used and is in immaculate condition. This does not interfere with the actual picture. I was wondering if this could interfere with the auto focus, but again, confuses me as to why it does focus in AI Servo mode or when I use live-view. Does this have something to do with the use of the mirror?

I am planning on testing the lens on a friends 350D to see if the AF behaves in the same way. At least it doesn't have live-view mode to confuse things.
 
Welcome to the forum.

Yes, the first thing I'd suggest, would be to try the lens on another camera (I assume your other lens work without problems?). If the lens behaves the same on another camera, then you'll know it's the lens. If not, the problem may be with your camera, so I'd suggest trying your camera with several other lenses, to see if you can replicate the problem.

Also, have someone else (who knows their way around a DSLR) to give it a try. Most of the time, problems like this are user error so if you can rule that out first, all the better.
 
We've figure out the problem with the lens, and it turned out to be the lens itself. The issue isn't with the Autofocus but the fact that the Aperture Diaphragm was stuck closed, which was noticable when it wouldn't change when selecting a high aperture and pressing the DOF Preview button. This will then be affecting the amount of light coming through the lens and stopping it from focusing properly. After a quick google, it seems that this is actually a fairly common fault with this lens where the Aperture ribbon breaks.

Thank you for your comments, time to replace the lens I think.
 

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