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02-09-2012, 01:01 PM   #1
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Auto aperture not working

I purchased a used Sigma 170-500 off ebay. When picking it up yesterday, visually it all seemed ok.

I've tried it on two K-x bodies with same results - it is a very stiff/tight fit to mount, needing that little bit more force than normally to twist, and the aperture doesn't display on the screen when in A mode. If I tap the lens a little, it will flash up and then disappear again. As though the contact pin isn't making contact. Running my finger over the contacts feels as though the aperture contact actually sit higher than the rest though. manually setting the aperture still works.

Could both issues be related - the tight mounting is affecting the contact?
Is it possible (for a novice) to adjust the mount to make it that little looser?
If I remove the ring to clean the pin contacts (sticky tape on outside on pins to keep in place as suggested elsewhere) are there any other little surprises behind the ring I should be aware of?
Any other suggestions please?

Thanks
Andrew


Last edited by Stray Dingo; 02-09-2012 at 01:12 PM.
02-09-2012, 02:28 PM   #2
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Before you go dismantling the lens, even partially, I'd clean the contacts with a pencil eraser first. Do not use an ink eraser. It is TOO abrasive and can damage the contacts. A nice pink pencil eraser is just abrasive enough to buff off any oxidation on the contacts. Do the camera's contacts as well, but be careful not to let any of the rubbings fall into the camera.

Does the lens turn all the way to the locked position, when you mount it? Do you need to use the lens release button to dismount it? If not, then it is not really mounting properly and that may be your problem.
02-09-2012, 03:54 PM   #3
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Thanks,
Cleaning the contacts was the first thing I did - although I used a microfibre cloth rather than pencil eraser. (I know everyone says a pencil eraser, but it is still a vegetable oil based product isn't it - aren't oil based products bad for connections?)
The cloth did bring a lot of gunk off, and enabled the intermittent flashing up of the aperture on the screen.

All other lens run ok, so hadn't done the camera - but yes, makes sense to give that a whirl also.

The lens definitely clicks into the correct position, and seats correctly.
02-09-2012, 04:32 PM   #4
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Tight fit & contact reading are different problems. Sometimes even Pentax made lenses have slightly tighter or looser fit due to slightly different mount thickness. But for lenses like 170-500m, the mounting should be on the loose side due to the weight and size of the lens (unless the lens mount is bent).

As to the contact pins, you can try alcohol. If that doesn't work, spray some WD-40 on a piece of tightly folded toilet tissue then rub all contacts a few time. This should improve the conductivity greatly. If that still doesn't work, then all contacts underneath the mount must be cleaned (again WD-40 should do). An US product called DeoxIT is great too but costly and doesn't necessarily work any better than WD-40.

BTW, I recommend against the use of any eraser due to their abusive nature. The contacts will rust eventually.

02-09-2012, 04:41 PM   #5
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I just got a Pentax D FA 50mm Macro lens yesterday, and while I initially had no problems with it, I am now experiencing a similar problem with it. While the camera typically displays the aperture correctly, it may intermittently flash F-- or drop out of live view. Any suggestions? (Review and serial number database entry not yet updated; if the problem persists, I'll change them.)

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02-10-2012, 12:31 AM   #6
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Ok....
I had a bit more of a play.....
When mounting the lens, it 'feels' as thought the flange closest to the contacts is the tight one. I also found the plastic inside the body was marked/rubbed at that point.
The rear cap supplied is different to any others I have, and not an original Sigma one. So I grabbed an original sigma one, and it too is very tight to fit. It also pushes away slightly from the flange closest to the contacts - which doesn't occur on my other sigma lenses. So gut feel is that that is the problem. I'm also thinking if that one flange is not quite right, it is forcing the contacts away so not engaging right, which is why if holding in a certain manner or tapping the lens a bit results in the setting flashing up and disappearing again. (yes this is all conjecture and I'm theorizing)

So lets say that is the case.......
Is it reasonable to assume if I could get my hands on another very cheap Sigma lens off ebay, say an old AF kit lens, I could swap the mount plates. (sigma since screw holes will align etc) or is that unlikely to match exactly?
02-10-2012, 12:37 AM   #7
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The mount can be exchanged, or you may even get one from Sigma I would guess. But what happened? Was the lens subjected to impact? If so. I would just seek for a refund because you never know what else might be damaged like optical misalignment.
02-10-2012, 04:10 AM   #8
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No idea. He 'says' it was was never dropped, and was fine on his K200, but really, who knows. The images taken seem ok, sharp, focusing etc.
I haven't ruled anything out - if I need to demand a refund, then I will. Just seeing what my options are first....

02-10-2012, 04:34 AM   #9
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Then maybe you can contact CR Kennedy and see how much to have it fixed.

Welcome to C.R. Kennedy | Photographic, Digital, Medical, Surveying, Surveillance, Digital Audio. CR Kennedy. Australia
02-10-2012, 06:13 AM   #10
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Just a few simple comments here.

tight fit of sigma lens caps, is not an example of an issue with the mount. Sigma lens caps for PK lenses are , to say the best, terrible. They are orientation specific and very snug on all lenses, so forget that point.

lens mounts can be very slightly different in allignment, and sometimes do not contact the pins, this is also not unique to sigma.

It may also be that the contact within the lens is dirty, not the external lens contacts. This has been a complaint from time to time on all lenses that sit in the A position for most of their life.

Try going in and out of the A position several times and see if it gets better
02-26-2012, 08:06 PM   #11
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Well, I ended up taking it into CR Kennedy. As soon as the tech looked at it, he said the mount was damaged. (I couldn't anything obvious, but they could). So two weeks and $143 later, the mount is replaced and the aperture setting is back. All good.

Thanks to all for replies
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