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04-10-2018, 04:39 AM   #1
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One lens won't do stop-down metering on both K-1 and K-5

It's a Cosina 28mm f2 lens. The lens simply refuses to close its aperture on both my digital bodies no matter what I do. I'm fully familiar with the entire stop-down metering procedure and it works as it should with all other manual lenses I own, but not on this one.

The lens closes its aperture just fine on my old Pentax MX.

Does anyone have a clue what could be the problem? The lens has a metal mount and it seems clean if that matters.

04-10-2018, 04:46 AM   #2
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Is the lens mount black anodised?
If so you will need to scape it off so the camera contact can work.


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04-10-2018, 04:57 AM   #3
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What I did with a lens that has a non-conductive, anodized mount surface, was to use some aluminum tape in the area where the camera "data" contacts strike the lens mount. I know not everyone has easy access to such tape and I wouldn't suggest that people buy a roll of it for this purpose. It's commonly used in the aircraft industry and taking an inch off a roll was not a problem in the hangar I was working next to.
04-10-2018, 05:28 AM   #4
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QuoteOriginally posted by pres589 Quote
What I did with a lens that has a non-conductive, anodized mount surface, was to use some aluminum tape in the area where the camera "data" contacts strike the lens mount. I know not everyone has easy access to such tape and I wouldn't suggest that people buy a roll of it for this purpose. It's commonly used in the aircraft industry and taking an inch off a roll was not a problem in the hangar I was working next to.

Probably a much better answer than scraping the lens mount. I bought some tape for the same reason but have not bought any further lenses since.
I can point out that the K20D and earlier bodies seem to be able to use black anodised lenses without any modification.

04-10-2018, 09:04 AM   #5
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You can obtain small quantities of copper adhesive-backed tape in varying widths on ebay. It should work fine for making a shorting path on the back of the lens (should that be the issue in your case). It's also available in some plant shops for controlling slugs and snails but is probably a more expensive option that way.

Last edited by Bob 256; 04-10-2018 at 09:10 AM.
04-10-2018, 09:42 AM   #6
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I don't think it's anodized. Here's how the lens mount looks like.

04-10-2018, 11:01 AM   #7
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Two guesses:

1) Is the lever which controls the aperture bent or at a slant? If it is the DSLR may not be making a good contact to move it, or
2) Is it stiff or sticky (try it with your finger)? I've had an old Pentax-M lens which the film camera could deal with but DSLRs found too hard to move

Sorry if repeating the obvious but there doesn't appear to be any electrical contacts there (common on older lenses) which could be causing the issue, so I'm thinking mechanical

04-10-2018, 11:36 AM   #8
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Oh well you have got me there, must be a mechanical issue as MrMojo has suggested. I do remember when I got my Cosina 50mm F2 that the aperture lever movement was stiff, the mechanism under the screwed on plate needed a tiny bit of lubrication to get it to move freely. Maybe your DSLR does not have the power to move it if you find yours similarly stiff.


04-10-2018, 12:24 PM   #9
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Well, this is becoming something of a mystery.

The lever is not bent nor slant, and it most definitely is not stiff nor sticky. Together with this lens I also bought a SMC K 28mm f3.5 lens and its aperture lever is markedly more difficult to move in comparison to the Cosina, and the SMC K still meters stopped-down without a hitch on my K-1.

Just to be sure, I also compared the length of the lever between the two lenses and the one on the Cosina is not shorter either.

Do you think it's strange that I can't hear any sound coming from my digital bodies when attempting to stop-down meter with this lens? It's as if the camera is not even attempting to move the lever regardless of whether I am pressing the green button or moving the On/Off switch to optical preview position. I can do the optical preview with the SMC 28 without any problems, as I can with any other manual lens that I own.
04-10-2018, 01:16 PM   #10
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Use double stick Scotch tape and aluminum foil. Carefully trim the foil, backed by the tape, to shape to cover the base of the lens mount where it covers the contact pins on the camera mount. Stick it on the lens mount. Mount the lens. Voila!
04-10-2018, 01:49 PM   #11
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Is the point to prevent the contact between the lens and the contacts on the camera?
04-10-2018, 01:52 PM   #12
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QuoteOriginally posted by deepbluish Quote
Is the point to prevent the contact between the lens and the contacts on the camera?
The point is to short the contacts with a conductive material.
04-10-2018, 03:00 PM   #13
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OK. But I still don't understand what is different between this lens and other manual lenses with metal mounts? The material? I can't see any difference.
04-10-2018, 03:21 PM   #14
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QuoteOriginally posted by deepbluish Quote
OK. But I still don't understand what is different between this lens and other manual lenses with metal mounts? The material? I can't see any difference.
When you mount the lens and turn the camera on, does the camera ask for FL Input? If not, the camera doesn’t even know a lens is mounted.

Is it possible the flange on the lens mount is preventing the lens body from contacting the body mount? Otherwise, for some reason the lens mount is not conductive.
04-10-2018, 03:38 PM   #15
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Yes, it does ask for FL input.
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