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03-29-2012, 06:01 PM   #1
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Help with SA/KPA mount

I bought a used Sigma 70 - 300 mm F 4-5.6 APO Macro lens online that I thought would fit my K-r, but it doesn't seem to. The lens has a rear cap that says SA/KPR and I went online before purchasing the lens to see what that meant. My research, which apparently was not comprehensive enough, consisted of determining that the electrical connection pins on the Sigma lens were in an identical location to the pins on my other Pentax lenses, so i figured the lens did not have the SA mount. Then I looked for the dreaded Ricoh pin, and found none, so I figured the mount was not a Ricoh, and therefore must be Pentax. The end looks the same as my other Pentax lenses with the exception that the small lever that sits outside the metal guard and opens the aperture is just a bit wider than it is on my other lenses. The lens seems to slip in to the opening on my camera when the red dots are lined up, but will not turn. I tried it first on a P3, then a ZX-7 body before trying it on my K-r. The results were the same. Can someone please assist me with what kind of a mount this is? I searched the forum and could not find any answer. Thanks!

03-29-2012, 06:08 PM   #2
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Can you post a picture of the mount?

The problem with identifying a Sigma lens is that they use the same rear cap for both the SA mount and the K mount.
03-29-2012, 06:30 PM   #3
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Photos may show the problem?

Here are two photos of the lens in question. The side view shows an apparent bend in the chrome rim between two of the flanges. If this is a Pentax lens, perhaps the flanges are slightly bent down leaving too thin a space between the flanges and the lens body, which could be why the lens won't turn to lock on to the camera. Just a thought, Perhaps someone with more knowledge on these matters will know if this is indeed a Pentax mount and why it will not turn to lock into place on the camera.
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03-29-2012, 07:01 PM   #4
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I posted two photos ...one of the end view and a side view showing what may be a bend in the mount. I don't see any scratches or other signs of damage, however, but if this is a Pentax mount, then the bend could be the issue. If that is the case, can you change the rear metal piece with the three flanges without too much difficulty?

03-29-2012, 07:10 PM   #5
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I just checked a few of my SA mount lenses (which have the same end cap) and don't match your picture. Don't mean to be upsetting but did you align the red dot on the lens to the red dot on the camera when mounting?
03-29-2012, 07:25 PM   #6
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Yes, I did very carefully align the red dots. If your lenses are SA mount, then they should not look like my lens, which I thought was a Pentax mount. Thanks for checking and responding.
03-29-2012, 07:39 PM   #7
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I'm glad you aligned the dots. You're adhead of the learning curve. : )
I checked a pentax af lens and it closely matches the sigma, however there is no bend in the chrome. could be a defect or a mishap dismounting the lens or cracking nuts. can you send it back? refund/exchange?
03-29-2012, 07:53 PM   #8
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It is definitely a Pentax mount.

This is what the mount should look like from the side.

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03-30-2012, 08:40 AM   #9
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So I did correctly identify the mount as a Pentax PK mount; the problem must be the apparent bend in the mount. Fortunately for me, the seller immediately agreed to stand behind the lens! Now, I wonder if that bent metal piece is something that can be removed and replaced without too much effort, or if it is part of the basic framework of the lens. I have never taken a lens apart, but may have to try.
03-30-2012, 03:11 PM   #10
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QuoteOriginally posted by sandstone Quote
So I did correctly identify the mount as a Pentax PK mount; the problem must be the apparent bend in the mount. Fortunately for me, the seller immediately agreed to stand behind the lens! Now, I wonder if that bent metal piece is something that can be removed and replaced without too much effort, or if it is part of the basic framework of the lens. I have never taken a lens apart, but may have to try.
It shouldn't be too terribly hard to take apart (be careful of pulling too hard on wire ribbons that send the electrical signals). The problem is going to be finding a replacement mount. They are not standard or generic. You have to find one to specifically fit that lens (or perhaps that and similar sigma lenses).
04-01-2012, 09:55 AM   #11
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I think I'll try taking the bayonet mount off. i have contacted Sigma, who referred me to their repair center in Chandler, AZ. They told me that Sigma does not service old lenses like my 70-300mm APO Macro, but they are trying to find out if they can get the new part. I will have to check the 2 other Sigma zoom lenses I have and see if this part is the same. If so I can cannibalize my 28-80 zoom if I can't pick up another. I'm surprised that Sigma would change the Pentax bayonet part with every revision in their lenses. So it goes in today's throw away world.
04-01-2012, 10:29 AM   #12
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In trying to determine the exact issue, I think it might be one of the bayonet lips would be bent inwards from impact damage. This would allow the lens to sit on the camera body but not rotate or only partially rotate. This can easily be measured with some gauge blocks and feeler gauges (standard machinists tools). IF, an big IF the mounting base can be removed easily, it should not be a big deal to have a machinist chuck it up in a small lathe and do an inside cut to reestablish the correct tang clearance. In other words removing the interference portion of the bayonet lip. Basically pretty simple if you can find a machinist friend and the mount removes easily. One other option that I would dislike would be to purchase a set of jewler's files and have a go at the inside surface to regain rotational clearance. If the mount was not removed from the lens, you would have to seal it up very well to avoid filings traveling inside and then clean same with a magnet and then air etc. A long shot idea.
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