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01-28-2011, 06:34 AM   #1
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M42 lens mount questions

Last fall I bought an H1a, mainly for nostalgic reasons (my first SLR back in 1968), and it's in very good condition except that I've noticed that lenses don't seem to screw on as far as they should.

They go on smoothly, the usual roughly 2 1/2 turns, until the register mark (the red diamond) gets somewhere in the range of 5-10 degrees from vertical, and they stop there. The lens is tight against the lens ring/faceplate of the camera, and there's no obstruction behind the lens. I've taken the faceplate off to look at the lock ring that holds the lens ring on, and everything appears to be okay. There's a small pin or bump on the back of the lens ring that seems to fit properly into a hole in the faceplate.

Do you think that 5-10 degrees affects infinity focus? Here's the link to a shot I made right after I got the camera and before I noticed this problem: http://www.flickr.com/photos/nparsons13/5392835275/. It doesn't seem to be as sharp as it should be. The lens in this case is a Meyer Oreston 50/1.8.

What do you think? Am I creating a problem where there is none? Should I loosen the lens ring and turn it 5 degrees more? Is that even possible, or is there only one position at which the lens ring fits?

01-28-2011, 07:32 AM   #2
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infinity focus is a function of lens flange to sensor/film plane and not the rotation of the lens

I find many of my M42 lenses do not allign perfectly vertically, but focus is not an issue.
01-28-2011, 07:38 AM   #3
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If you have an M42 adapter for your DSLR you could check the lens there.
Basically I would say that 5-10 degrees wouldn't matter much.
01-28-2011, 08:29 AM   #4
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My Mamiya/Sekor 50mm f/2 is the same way... I wouldn't worry about it.

01-28-2011, 08:32 AM   #5
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I've only got 1 m42 lens that aligns perfectly, the rest come up a little short but doesn't seem to be a problem...
01-28-2011, 08:50 AM   #6
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I will join the chorus...
  • Lens registration is determined by the flange. As long as the lens is snug on the mount, there should be no issue.
  • Very few M42 lenses line up with the focus index mark top-dead-center on the camera, regardless of maker or the mix of body/lens.


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01-28-2011, 09:37 AM   #7
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Thanks to all of you for your reassuring replies. I'm not going to let it bother me anymore.

I've tried several different M42 lenses with a Pentax adapter ring on a K200D and a K1000. Takumar lenses line up closer to TDC on all three cameras than the lenses of other brands. On the K200D most of the lenses line up better than they do on the H1a, and on the K1000 nearly all of them line up. All these years, and I never really noticed that variation between cameras before.

The lens that is farthest off appears to be an old 35mm 2.8 Tamron with an Adaptamatic mount. It's obviously off even on the K1000.

Thanks again.

Noel

Last edited by nparsons13; 01-28-2011 at 10:26 AM.
01-28-2011, 10:44 AM   #8
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Not sure I am in the right thread for this, but just let me know if not. My question: I've read that the best adapter for a screw mount lens on my K200D would be an original Pentax made adapter...ok..but how to differ it from a copy ?? I found an original one on ebay for $50 ( still in the package), while there are so many other ones for $10-20 ??? .... I don't mind to pay $50 if this is the cost. Any help ?

01-28-2011, 11:11 AM   #9
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QuoteOriginally posted by cdnkit Quote
Not sure I am in the right thread for this, but just let me know if not. My question: I've read that the best adapter for a screw mount lens on my K200D would be an original Pentax made adapter...ok..but how to differ it from a copy ?? I found an original one on ebay for $50 ( still in the package), while there are so many other ones for $10-20 ??? .... I don't mind to pay $50 if this is the cost. Any help ?
Original is sold through Amazon or Pentx site: PentaxWebstore Mount Adapter K(35m m Screwmount to Bayonet) - Ebay is kinda odd place to shop if you're not familiar with the product... be careful.

This is the one from Amazon, where I purchased mine: http://www.amazon.com/Pentax-30120-Mount-Adaptor-B/dp/B00005ABA6/ref=sr_1_11...241629&sr=8-11

Last edited by theunartist; 01-28-2011 at 12:08 PM.
01-28-2011, 11:55 AM   #10
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Chorus: flush contact between lens mounting face and front face of camera mount place lens at correct registration distance from film/sensor for correct infinity focus regardless of radial position.

A mis-focus problem will be more evident with wider angle lenses because the movement required to change focus from minimum distance to infinity is smaller than the movement required for longer lens. For example, a mis-focus problem undetectable with 200mm lens may be quite evident with 20mm lens.
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