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Old Nov 18, 2006, 12:15 PM   #1
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On my 20D.

When I push my shutter half way to get focus lock in one-shot mode the camera seems to hunt backwards and forwards very rapidly before settling on a spot that seems to be slightly off the correct focus, and is slightly front-focussed.

It seems to be doing it primarily with my 17-85mm lens.

I have tried two batteries both of which were showing good charge, but one of which probably had only 1/2 charge and the other which should have had a full charge, but has been sitting in my bag for a month or so. So I cannot rule out a battery problem. And of course both batteries are now a little over 2 years old.

If I switch the lens to manual focus the focus indicator seems to light up when I have correctly focussed the picture to confirm focus, and is (as best I can tell) not lighting up when I haven't got the focus correct. The AF indicators concur with what my eyes are telling me, but when AF is switched on it misfocusses.

The problem has only occurred twice, both times outdoors in chilly (~8C) weather, but that shouldn't be cold enough to be a problem I don't think.

Thinking back now I have only seen this happen when focussing on far away objects, not quite at infinity, but close to that end of the focus adjustment.

It is working fine again indoors for near objects, and of course it's warm inside.

I don't think this is a generic AF issue, it was working fine and then suddenly started this weird hunting and was bad enough that I had to switch to MF. I've had the camera for 2 years, so I know that it usually works very well under these conditions, and the subject matter was not one that has ever given the camera pause before.

It happens both with the single AF point selected and with all AF points selected.

The contacts between the camera and lens are both clean and free of corrosion. I did remove and re-attach the lens, because I thought it might be that the connection wasn't right. There doesn't seem to be any undue movement at the mount. It shifts a little bit if I wiggle it, but it doesn't seem unusual in that regard.

Any thoughts??

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Old Nov 18, 2006, 2:00 PM   #2
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It sounds like you may have a 'sticky' secondary mirror...

-> The primary mirror only affect the manual focus whereas the secondary mirrors redirect the image to the AF sensor below the film chamber - of course cold weather may make it more sticky than warm when the lubricant 'gel'.
The AF confirmation light may be a red herring since it could light up within the DOF...
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Old Nov 21, 2006, 1:59 AM   #3
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Thanks NHL.

I'll continue to try to narrow it down a bit, I hope it holds out until March '07, when the 5D upgrade comes out.

I guess sending it in for a service might sort things out.
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