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04-19-2016, 01:36 PM   #1
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Pentax-M 20mm f/4 focusing issues

Hi!
Today I was given a Chinon CE-4, which isnt a very interesting camera. But attached was a Pentax-M 20mm f/4 which got me very exited.
I cleaned the lens which turned out to be in great condition. BUT, when looking through the camera it does not focus to infinity!
I looked over the lens and it look fine to me, I even tried on 4 diffrent cameras to make sure it was not the Chinon that was wrong.
It went from the Chinon to my LX, and from there to my MX and K-3. Does not focus. Do you have any suggestions? I heard from a
guy in facebook who said it could be the helicoid. Please see the pictures below

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04-19-2016, 01:53 PM - 1 Like   #2
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You'll probably need to open it and set the infinity stop.
I'm not an expert, but those lenses of ye old M series should be quite easy to take apart and put together again.

Just wait for someone to chime in, in the meantime search for something like "adjusting infinity focus" and search for the disassembly instructions for that lens on this site or elsewhere on the internet.
04-19-2016, 03:50 PM - 1 Like   #3
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What he said ^^^. I had the same problem with that lens which I used to own. It was easily adjusted for infinity focus. https://www.pentaxforums.com/forums/58-troubleshooting-beginner-help/205733-p...-focusing.html
04-19-2016, 04:13 PM - 1 Like   #4
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When you say it does not focus up to infinity, do you mean that when you turn the focus ring, it does not line up with the diamond shape focus-marker? This can by adjusted by loosening up and re-aligning the 3 screws covered under the name ring, which you have to remove first.

04-20-2016, 12:32 AM   #5
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QuoteOriginally posted by pixie Quote
What he said ^^^. I had the same problem with that lens which I used to own. It was easily adjusted for infinity focus. https://www.pentaxforums.com/forums/58-troubleshooting-beginner-help/205733-p...-focusing.html
How did you fix it? Can you give a tutorial? Thanks for the link!

---------- Post added 04-20-16 at 12:34 AM ----------

QuoteOriginally posted by aleonx3 Quote
When you say it does not focus up to infinity, do you mean that when you turn the focus ring, it does not line up with the diamond shape focus-marker? This can by adjusted by loosening up and re-aligning the 3 screws covered under the name ring, which you have to remove first.
The focus ring moves as it should, and reaches the infinity mark. Igs trough the camer that it does not focus :/
04-20-2016, 08:38 AM   #6
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Two things that I would like to point out especially about wide-angle manual focus lens, which this M 20mm lens fits into that category.

First, is the hyperfocal distance for a wide-angle lens to obtain focus (up to infinity). With this lens, it is not a far distance away from the point where you hold the camera. What this means is that in normal use, you don't really need to set turn the focus to infinity, in fact rarely; of course, unless you are trying to take a shot of the moon. But then again, you will be using the wrong focal length lens (should be a very long focal length) for this usage. To assist you with the achieving the hyperfocal distance, there is a DOF marker at a given focal length in the M lens (in fact in all manual focus lenses), .

Second, depending on which camera you use, there is AF fine adjustment setting that you can try to fine-tune the focus (applies to all setting) if you are using PDAF via the viewfinder. Alternatively, you can try to use Live view (back LCD) focus peaking (if your camera has this feature) to nail the focus.

---------- Post added 04-20-2016 at 11:41 AM ----------

QuoteOriginally posted by henrikknutsson Quote

The focus ring moves as it should, and reaches the infinity mark. Igs trough the camer that it does not focus :/
When you say it does not focus, does it mean the "green hexagon" looking through the viewfinder does not appear? That could be some fine-adjustment to be made in the PDAF mechanism in the camera. Did you try using Live view to focus as well?
04-20-2016, 09:04 AM   #7
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QuoteOriginally posted by aleonx3 Quote
Two things that I would like to point out especially about wide-angle manual focus lens, which this M 20mm lens fits into that category.

First, is the hyperfocal distance for a wide-angle lens to obtain focus (up to infinity). With this lens, it is not a far distance away from the point where you hold the camera. What this means is that in normal use, you don't really need to set turn the focus to infinity, in fact rarely; of course, unless you are trying to take a shot of the moon. But then again, you will be using the wrong focal length lens (should be a very long focal length) for this usage. To assist you with the achieving the hyperfocal distance, there is a DOF marker at a given focal length in the M lens (in fact in all manual focus lenses), .

Second, depending on which camera you use, there is AF fine adjustment setting that you can try to fine-tune the focus (applies to all setting) if you are using PDAF via the viewfinder. Alternatively, you can try to use Live view (back LCD) focus peaking (if your camera has this feature) to nail the focus.

---------- Post added 04-20-2016 at 11:41 AM ----------



When you say it does not focus, does it mean the "green hexagon" looking through the viewfinder does not appear? That could be some fine-adjustment to be made in the PDAF mechanism in the camera. Did you try using Live view to focus as well?
Since I have mounted it on both film and digital cameras I dont think that is the problem. I took some shots with the K-3 and the lens set to infinity, and it was really blurred out.
04-20-2016, 09:15 AM   #8
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QuoteOriginally posted by henrikknutsson Quote
Since I have mounted it on both film and digital cameras I dont think that is the problem. I took some shots with the K-3 and the lens set to infinity, and it was really blurred out.
Interesting... perhaps other users may have some idea, I am puzzled.

04-20-2016, 09:19 AM   #9
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QuoteOriginally posted by aleonx3 Quote
Interesting... perhaps other users may have some idea, I am puzzled.
I really think the helicoid needs calibration, as some people recommended on other forums and Facebook. Should be an easy fix for a experienced repairman.
04-20-2016, 04:57 PM   #10
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QuoteOriginally posted by henrikknutsson Quote
How did you fix it? Can you give a tutorial?
I did not fix it myself, I had it repaired.
04-20-2016, 07:30 PM - 1 Like   #11
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I have a K28/3.5 that would not focus to infinity. Eric Hendrickson adjusted the colimation (helicoid setting) and it is now superb. IIRC if was less that $50 for a the repair.
04-21-2016, 12:34 AM   #12
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QuoteOriginally posted by monochrome Quote
I have a K28/3.5 that would not focus to infinity. Eric Hendrickson adjusted the colimation (helicoid setting) and it is now superb. IIRC if was less that $50 for a the repair.
Thanks! I will remember his website
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