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Renato1
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Veteran Member
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Posts: 3,385
Re: Sigma 400mm Lens Goes Haywire.
SQLGuy wrote:
You should be able to check whether there's a stripped gear.
If you turn the focus ring with the lens detached from the camera, you should see the screw drive head in the breech turning. If it's not turning when the focus ring is turned, then something is certainly damaged in the AF drive mechanism. If it is turning, you can put a small screwdriver in the slot and put LIGHT resistance on it while turning the focus ring. If you that the screwdriver starts slipping or things feel really gritty, that would probably indicate a broken or stripped tooth.
I didn't see anyone selling this repair part, and Sigma seems to be pretty bad about parts availability for their older lenses... but it would probably be something that could be 3D printed.
Thanks very much for your input.
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I've done as you said, and when I turn the focus ring the screw drive head turns too.
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But the turning of the focus ring isn't smooth at all - it gets lightly stuck three times through the mid to long focus part of the range requiring slightly more pressure to keep it turning. And it gets more stuck once at the closer focus part of the range - requiring considerable pressure to keep turning it at that point. Which I think explains where the problem is coming from, and why the KM5D seems to work with it some of the time. Though why its autofocus won't function at all in the A700 remains unexplained.
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I have 400mm Tokina SD lens attached to the A77 at the moment, so even if the Sigma isn't repairable, I can still always use it as a manual focus lens in a NEX camera.
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Regards,
Renato