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07-02-2020, 04:18 PM   #1
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What is this loose ring in the back of my Pentax-M 100mm 2.8 and how do I fix it?

This forum has been such a gold mine of knowledge that I hoped someone here would be able to help with another gear question. I bought the Pentax-M 100mm 2.8 on FleaBay and as I focus and head toward infinity, I noticed it would stop and jam up. Then I shook it an noticed the rattling sound and that there was a ring between the front element of the lens and the mount that was loose. I can get it back in place but if I adjust focus again, it shifts and I'm not able to change the focus until I realign the metal ring internally. Any idea what that ring is and to fix it? Do I just return the lens? Uploaded a video here to hopefully help illustrate the issue:


07-02-2020, 05:19 PM   #2
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I'm not sure what ring you are referring to. All I see is the normal movement of the rear element and part of the helical thread follower.

If it is one of the slotted and threaded retainer rings tighten it down with a lens spanner wrench.
07-02-2020, 05:19 PM   #3
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That ring (wide outer one) isn't supposed to be loose. The narrow inner one should move with focus action in the fixed outer ring which sits flush with the mount.
07-03-2020, 06:36 AM   #4
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Out of curiosity are you watching with the volume up? It's maybe easier to actually hear how the lens jams up as I change focus because of an interior ring misalignment. Definitely does not seem normal as I can’t got from near to far without gingerly rotating in hopes the treads align. I’ve tried to better capture it here though it’s tough to shoot one handed.


07-03-2020, 07:20 AM - 2 Likes   #5
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I didn't check my M 100 2.8 lens; but from your second video, that sound is not healthy and the ring is askew. I would return it.

Added: After writing the above, I thought it better that I have a look at my own lens. That circular piece which is askew and moves in and out as you focus the lens should not even be moving. There is a very narrow and deep ring affixed to the rear element which moves in and out when you focus the lens; but that wider ring which I see in your video should stay fixed to the rear mount. Your lens is definitely broken and should be returned.

Last edited by cpk; 07-03-2020 at 07:33 AM.
07-03-2020, 09:05 AM   #6
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Thank you! Seemed wrong and if it canít be easily fixed, then I will return it.
07-04-2020, 04:54 PM   #7
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QuoteOriginally posted by acoulter Quote
Thank you! Seemed wrong and if it can’t be easily fixed, then I will return it.
Please let me know what happens in the end.

07-04-2020, 09:32 PM   #8
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QuoteOriginally posted by cpk Quote
Please let me know what happens in the end.
The Ebay seller was apologetic and sent me a refund without any issue and told me to keep the lens. It's sent me down the YouTube rabbit hole and I have ordered a lens spanner to see if I can get in there and fix or it or at least learn about the lens construction with this lovely piece of glass.
07-05-2020, 06:38 AM   #9
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It is a lovely piece of glass. Good luck with it. And that is very good of the Ebay seller.
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