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Hi, I've a Sigma 28-105mm aspherical 3.5-5.6 lens. Auto focus does not engage. I can hear the motor trying, but the ring does not turn. When I switch to manual focus, I can turn the ring to focus manually, but I still hear the motor trying to use autofocus. Additionally, sometimes the shutter will not fire in manual focus while the motor is still searching. I've cleaned the contacts, but no joy. I suspect a faulty switch, but that doesn't completely explain the autofocus not engaging. I am in the USA. No warranty is on the lens. I'm an experienced camera user and I've taken apart a couple dozen lenses and a dozen or so different cameras from old rangefinder Leicas to Nikon D90 and Pentax 6X7, so I'm far from a newb. I'm just not familiar with the Sigma autofocus motor and ring system.

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Well, I think you have at least one problem pinpointed, in that I suspect the focusing ring is jammed, and cannot rotate with the electromagnet signals. The other problem might be a faulty auto/manual switch. Of course, check for oxidation and excessive dust on the switch contacts, if you can get at them, and clean the fingers on the microprocessor with some alcohol, toothbrush, and clean cloth.
I would start by spraying the lens assembly the best you can with some compressed air, then bumping the camera gently, in case the focusing ring is just stuck.
Here's a good blog about stuck lenses and the resultant (sometimes) errors that ensure:
http://camerarepair.blogspot.com/2007/12/fixing-lens-error-on-digital-camera.html
With your mechanical ability, you'll have a good chance of fixing this, I think, but it is kind of tricky.
Good luck, not sure I told you anything you didn't already know...

Posted on Aug 01, 2010

  • bc557 Aug 02, 2010

    After further inspection, I've also noticed that the zoom ring slips at/near full extension (105mm). The lens then has to be pushed back in manually before it will catch again. It moves just fine, back and forth both, between 28-80 mm. The interesting part here is that after the zoom ring slips it will rotate quite some distance, and near the end of its rotation it will *also* catch the focus ring and rotate it simultaneously.

    Guess: someone probably tried to turn the focus ring manually while the camera was set on AF. Are the pins on this lens for both the focus ring and the zoom, or are they independent? They seem to be interconnected.


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