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11-18-2012, 08:11 PM   #1
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Pentax-M 20mm, question about focusing

Hello everyone,

I got a M-20mm on ebay last week. I don't seem to be able to make it produce in-focus shots of anything farther away, let alone infinity. Close focused shots come out nice. Anyway, even at f/8 and set to infinity, the horizon comes out blury.

I still hope the problem lies with me and not the lens. Any ideas? Oh, I have noticed the aperture blades are not so snappy, but that alone couldn't be the culprit, right?


Last edited by pixie; 08-23-2014 at 11:47 PM.
11-18-2012, 10:33 PM   #2
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Is the second picture with the focus set to infinity?
11-18-2012, 10:43 PM   #3
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yes, it was, and so was the third picture
11-18-2012, 11:27 PM   #4
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The focus could be out of calibration with the ring on the outside, look through the view finder and follow the in focus area from close focus and see when (or if) it gets to infinity and what distance that corresponds to on the focus ring.

11-18-2012, 11:32 PM   #5
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First, make sure you're not actually going past infinity-- trying backing off from infinity on the ring just slightly and stopping down. Usually if a lens isn't quite reaching infinity, if you stop down far enough it will become in-focus (more than you have there), but that is not necessarily true of a lens that goes past infinity. Either way, probably needs an adjustment to get it right, which is usually fairly easy on M lenses and Takumars, but those extreme wide-angles have a different construction -- not sure if it is simple or not with that one. (Usually involves taking off the name ring on front, loosening some screws, adjusting, re-tightening -- repeat as necessary.)

But it could be some other problem -- you might consider just returning it and avoiding the hassle.
11-18-2012, 11:51 PM   #6
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Thanks for your help, guys. The camera won't indicate focus past about 4 feet. Until there everythings is fine. How expensive do you think fixing this would be or is it even possible. I was so excited about the lens when I got it, especially because it looks pristine on the outside... Also, the lens will focus at 5 inches or 12cm, I believe the specs call for 25cm as minimum.

Last edited by pixie; 11-19-2012 at 12:05 AM.
11-19-2012, 12:17 AM   #7
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It does sound like someone has pulled the lens apart and put it back together with the focus elements not properly calibrated to the focus ring. If that is the problem, you can try fixing it yourself, take it to a repair shop or maybe someone from here will do it, but without seeing the lens it is hard to be exact. If you really like the lens I suggest taking it to a trained repair professional first at least to make sure that is the fault. Then decide if you wish to fix it or return.
11-19-2012, 06:04 AM   #8
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And this is a Pentax brand M-20mm lens (sample shown below)? It almost sounds like this is a lens on a mount adapter - that would explain the ability to focus closer than spec (the mount adapter acting like an extension tube) and not reaching infinity.


11-19-2012, 07:35 AM   #9
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Same lens, no adaptor, directly on a K20D
11-19-2012, 07:43 AM   #10
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QuoteOriginally posted by Pentaxpixie Quote
Same lens, no adaptor, directly on a K20D
Sorry, needed to ask to make sure none of we armchair technicians weren't chasing down the wrong paths.
Given that it is the real deal, it sure sounds to me like at least one element is out of place. As suggested by vonBaloney ... repair shop or return.
11-19-2012, 08:37 AM   #11
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Absolutely, thanks again everyone.
11-19-2012, 10:17 AM   #12
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An element reversed is a possibility -- taken apart and put back in upside down. That can be more subtle than you'd expect.
11-19-2012, 05:59 PM   #13
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QuoteOriginally posted by vonBaloney Quote
An element reversed is a possibility -- taken apart and put back in upside down. That can be more subtle than you'd expect.
Lol that's an interesting fault, should do that with an old m42 and see the result.
11-20-2012, 06:49 AM   #14
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Louicio - Been there done that, didn't work. Had a Lentar 300mm with fungus in rear lens. Very easy to disassemble, just unscrew rear barrel and lens, clean put back together. No doubt something was seriously wrong...but since that was the only thing removed, easy to figure it out. But...no infinity focus so this could be similar. Unfortunately since this one was done by someone else, if that's it, could be any element in it. My Lentar was easy, one side very convex, other side almost flat, I simply wasn't paying attention. I usually use a fine tip sharpie and put an arrow facing forward on the outside edge of any lens I remove as a reference for reassembly. Also didn't do that in that case...not the lens itself, the outside rim, which is usually ground but not polished.
11-20-2012, 09:23 AM   #15
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It is much more likely to need a simple infinity adjustment, the only odd thing that OP indicated was that it seemed like infinity was never in focus, even stopped way down. But when you go past infinity that often causes weird effects so that could be it. But certainly infinity adjustment is what I'd be looking at before assuming the elements are all screwed up. But if I had just bought it, I'd probably just return and find a better one...
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