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09-01-2015, 05:21 AM   #1
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K mount to M42 extension tube

Hi gurus,

I got a extension tube as free gift sometime back from a fellow pentaxian, when I bought a lens from him in this forum. Even after 5 years, I never used it, as honestly, I do not know how to use it and I have a FA50mm f2.8 macro for real purpose. Today, I took it out and found it interesting. It has regular K mount that fits the camera well and at the other end, it has screw-in lens facility, but not regular camera like mount. I think, it is M42 mount. I do not have any M42 lenses to confirm it. My question is, if I get a M42 lens, will I be able to use it as a regular lens by attaching it to the extension tube set or will it be macro only setup?? Please suggest. Thanks!

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09-01-2015, 06:18 AM   #2
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Several modern k mount extension tube sets are made as follows: k mount (male) ring to body with screw threads, then three different length tubes that you screw together with the first part, a final ring that is a k mount (female) that screws on the end. These tubes are not m42 but step rings might work to adapt them.

I have never seen a ring that natively has k on one end and m42 on the other. Typically an adapter is added. Also the modern k mount tubes use a threading that is larger than m42 since k mount itself is larger.

If you were to use the ring with step rings or if by a miracle it just happens to be 42mm correctly threaded for m42, it will only work as a macro tube since it will mount the lens such that it is pushed out from the original distance to the film/sensor. Even tiny changes to this distance render lenses unable to focus to infinity.
09-01-2015, 06:31 AM   #3
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It's not an extension tube. It's a T-ring for mounting T-mount lenses. Don't try to screw in a M42 lens, you will probably mess up the threads as they have a different thread pitch.
09-01-2015, 06:45 AM   #4
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QuoteOriginally posted by boriscleto Quote
It's not an extension tube. It's a T-ring for mounting T-mount lenses. Don't try to screw in a M42 lens, you will probably mess up the threads as they have a different thread pitch.
Well that makes sense also!

09-01-2015, 07:34 AM   #5
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T-rings (aka t-mounts) don't have a silver button on the side. Need a side-on pic... could be a t-mount on a short extension tube. check the thread: t-mount = m42 x .75 pitch; M42 = 1mm pitch.

More info:

How to use t-mount, preset lenses.
09-01-2015, 07:52 AM   #6
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Now that I can see it better (computer vs. phone) I am more confused. The inside surface on the first pic looks like the aperture linkage used for M42 Auto tubes (which isn't relevant on a digital camera) but the front is too hard to see. More pictures please. Also if you have a caliper or something to measure the threads that could help. Lastly does the K mount part unscrew from the rest?
09-01-2015, 07:55 AM   #7
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It could be a short M42 tube with a flanged K-mount adapter.

09-01-2015, 08:06 AM   #8
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QuoteOriginally posted by boriscleto Quote
It could be a short M42 tube with a flanged K-mount adapter.
In that case the K part would unscrew and look something like this: FotodioX M42 Pro Lens Adapter for Pentax K-Mount Cameras
09-01-2015, 08:38 AM   #9
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Sorry, I confused everyone. I think, it is a t-mount adapter attached to a K macro extension tube. I checked it carefully, depressed the silver pin and it came out. See the attached pic.
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09-01-2015, 08:49 AM   #10
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It is a cheap extension tube -- ebay is full of them. They are generally a PITA.
09-01-2015, 09:01 AM   #11
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Just to give ideas... I used M42 (genuine Pentax) which are cheap and available and an M42 adapter (flanged is fine) with a reversed lens and step rings to do some things. It's very inexpensive and opens lots of mounts for the lens that might be otherwise impossible.
09-01-2015, 09:14 AM   #12
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The smaller ring being held in the third photo looks like the small ring out of cheap 3rd party M42 extension tube set.
09-01-2015, 01:56 PM   #13
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QuoteOriginally posted by ntrao Quote
Sorry, I confused everyone. I think, it is a t-mount adapter attached to a K macro extension tube. I checked it carefully, depressed the silver pin and it came out. See the attached pic.
Are the threads male or female? They look like male threads in that photo. What is the diameter of the threads? If they are male and 49mm it is a K mount reversing ring.
09-01-2015, 02:03 PM   #14
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QuoteOriginally posted by boriscleto Quote
Are the threads male or female? They look like male threads in that photo. What is the diameter of the threads? If they are male and 49mm it is a K mount reversing ring.
Could also be 52mm and be a reversing ring. Several options available.
09-01-2015, 02:11 PM   #15
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I'm confused at what we are looking at. Is the separate ring in the 3rd photo the one with the K-mount bayonet? Or is it the ring on top in the second photo - connected to the ring with the silver knob?

Maybe a picture of all the separate pieces side by side and than another photo with them in the same relative positions but flipped over?
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