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09-30-2010, 03:51 PM   #1
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AF problem with Sigma 70-210mm F4 - 5.6

I have a 96 vintage Sigma 70-210 F4-5.6 lens that I've been happily using on my 1stDL for a few years. The AF has stopped working, there is tension from the motor when the camer is in AF mode but the indicator shows MF in the viewfinder. Ive tried the lens on my K20D with the same result. Ive also tried cleaning the contacts with a pencil rubber but no avail.

Have I missed anything that I should be checking and are these lenses worth repairing ?

10-03-2010, 03:22 PM   #2
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QuoteOriginally posted by adelante Quote
I have a 96 vintage Sigma 70-210 F4-5.6 lens that I've been happily using on my 1stDL for a few years. The AF has stopped working, there is tension from the motor when the camer is in AF mode but the indicator shows MF in the viewfinder. Ive tried the lens on my K20D with the same result. Ive also tried cleaning the contacts with a pencil rubber but no avail.

Have I missed anything that I should be checking and are these lenses worth repairing ?
Hrm, for thoroughness' sake, do you have other AF lenses that still work?

I suggest that before you try anything else, ...be sure of fresh batteries, and maybe even clean those contacts again. I'm not sure what would make the VF display claim MF... but it sounds as though the lens mechanism is jammed. (You might try jiggling or turning the screw drive connection on the lens, in case it's just stuck by some odd circumstance.)

I've never had an AF lens apart, so I don't really know how easy they might be to service: but odds are it may be cheaper to replace than pay to repair a lens like that, unless it's something very simple and you really like that lens. Generally, it'll be as costly to repair a cheaper lens as a fancier one, and lenses like that are pretty common.
10-03-2010, 11:01 PM   #3
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Thanks Ratmagiclady, you made me have another good look and I found the problem. There's a strip contact in a slot on the lens that connects across two contacts in the camera. A quick scrape (holding the the lens up so the particles don't enter the lens) with a small blade and the lens works good as new.

Last edited by adelante; 10-12-2010 at 12:38 AM. Reason: Wrong information
10-12-2010, 12:45 AM   #4
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Oops , I'm a goose. The information I gave is totally wrong. The two contacts I mentioned inside the camera are for a power zoom and the contact turned out to be the aperture actuator. Top top it all off the lens has stopped autofocusing again. I have 4 other lenses that are working fine so somthing is up with this one.

Does anyone know wich contacts pertain to the autofocus?

10-12-2010, 05:43 AM   #5
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K-Mount Evolution, Features and Operation

Features and Operation of the Kaf Mount

Features and Operation of the Kaf2 Mount
10-13-2010, 12:08 AM   #6
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Thanks for the information, It's interesting to see how the system works.

I haven't had much luck so far. I carefully removed the 4 screws on the bottom and removed the mount. I had a look at the contacts and their connections and everything seems ok, the only clue I have is the MF light is on when the camera is in AF mode but the af motor is on and giving resistance.

This gives me the impression that one of the contacts is open circuit, (same result with no lens attached) a schematic would be so nice. The lens works fine in MF mode with the aperture working ok so I'm confident that the digital contact is ok.

I'll look into it further when I have some more time but any clues would be greatly appreciated.
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