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02-05-2011, 10:23 PM   #1
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LTM Mirror Clearance?

In a mad fit of LBA, I recently purchased (ebay, of course) what was listed as an "extension tube" for M42. Well, did I get an interesting surprise. It is not quite that--it is actually a Leitz 13.5cm lens with all of the optics gone and the front part as well. Amazingly, the front part is thus left with an M42 fitting. But the rear mount is LTM (Leica Thread Mount, a form of M39), and I was wondering if the rangefinder coupling (e.g. http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/RF-Nikko...nRF85mmf15.htm ) will hit the mirror of a Pentax DSLR or not. The maximum extension of the metal piece is about 3 or 3.5mm past the end of the threading. So can anyone tell me if this protrusion will hit the mirror, or is it safe? Also, will that metal piece being inside the mount intersect the image circle and thus affect IQ?

If it's fine, then I can use it as a variable extension tube with my LTM-M42-K adaptors. And if not, I need to contact the seller....


Last edited by macTak; 02-05-2011 at 10:40 PM.
02-05-2011, 11:42 PM   #2
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When I tried a Jupiter-8 on my Pentax K10D, it worked fine. I think a lens would have to extend quite a bit more than 4mm to hit the mirror of a Pentax camera. You can check the clearance of the mirror on your camera by firing it with no lens mounted and paying attention to the mirror movement.
02-07-2011, 01:45 PM   #3
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LTM (alternatively L39) has a register of 28.8mm...M42 is 45.46mm. So, you won't get anywhere infinity focus. Just guesstimating, it will focus out to like 3 feet. And a lens with more extension away from the sensor will focus closer, making it a macro lens.

Plus, LTM is a weird thread pitch. I guess I can't imagine what you're trying to do with it on an M42 adapter...since it would have to be PK -> M42 -> LTM (or M42 -> M39x1 -> L39)

To answer the mirror question: My Sears 55/1.4 has a very large rear element. It protrudes 7mm from the mounting flange at infinity. I believe the Takumar 50/1.4 sticks out further yet; I can dig it out and measure it if you like.

The flange (as show on the Canon lens) won't be in the optical path--remember, the old lenses had to cover the full 24x36mm frame...you're just using the center
02-07-2011, 11:43 PM   #4
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QuoteOriginally posted by ryan s Quote
Plus, LTM is a weird thread pitch. I guess I can't imagine what you're trying to do with it on an M42 adapter...since it would have to be PK -> M42 -> LTM (or M42 -> M39x1 -> L39)
Thanks to you (and the other posting) for confirming the clearance is safe. What I'm going to do is be mounting some of my M42 lenses (most notably several macro lenses) on the front of it (which is M42) for some extreme macro photography (then on the back it will be LTM->M42->K. I already had the adaptors anway so I thought it might be intersting. I also may try some of my LTM enlarger lenses on the front (with another LTM-M42 adaptor)--with the right focal lengh (probably 105 or 135mm) I should get infinity focusing and a proper lens.

I did say it was a mad fit of LBA...

02-08-2011, 03:58 PM   #5
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Sounds like a lot of effort for essentially extension tubes and there's plenty of 135s out on the market

But, go nuts...especially if you have a bunch of stuff already sitting around. Why not?
02-08-2011, 11:26 PM   #6
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QuoteOriginally posted by ryan s Quote
Sounds like a lot of effort for essentially extension tubes and there's plenty of 135s out on the market
Actually, I got it for only a couple of dollars, so it was cheeper than any extension tubes (I would probably need two sets to get this much extension anyway). There are not plenty of 135's on the market thanks to yours truly (actually, I only have 3 135mm primes, plus the M120 and M150 )
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