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10-20-2011, 11:42 PM   #1
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Pentax F50mm f1.7 infinity focus.

Hi again. I have a really nice Pentax F50mm f1.7 lens that focusses perfectly on Karens K7, and my K2000. But it runs into the focus limit on my K5 at infinity(I can hear it klunk against the stop) and many times does not acheive infinity focus on that camera. I think it is a case of both the tolerances on that lens and that camera being just a bit too tight. I know the lens can be adjusted so it goes past the infinity a bit, but does anyone here have instructions for it on an AF lens like this one? Or who to send it to to do it? I have used CRIS in the past for this same issue on the 12-24 DA and it came back and worked perfectly after the adjustment, but I would like any advice of other vendors if I need to send it out so I have some options. Thanks again!!


10-21-2011, 01:02 AM   #2
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Sounds like your K-5 is not properly calibrated in the factory. The easy fix would be to have the lens adjusted. I don't know where but recommend against diy if you have to ask. There is a fragile contact brush inside which will most certainly be destroyed if you didn't do it before, and Pentax no longer carry this part. Good luck on your repair but worth it as good Pentax primes are getting rarer.
10-21-2011, 12:19 PM   #3
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I think it's pretty easy to do it yourself, with two cautions. Focus to the minimum distance, and you'll see three screws exposed on the barrel above the focus ring. Warning #1, the screws are JIS, so a Phillips #000 screwdriver almost fits but sometimes just ruins the screw without budging it. Remove those screws and the filter ring comes off like this:

Focus back to infinity. Just inside the focus ring and outside the front element, there are three screws that attach the focus ring to the rest of the lens. Warning #2, don't remove them. That puts the contact brush in danger as mentioned by wlachan. All you have to do is loosen them a bit until you can turn the focus ring without the lens actually changing focus. In your case, it sounds like it just needs to be turned a bit clockwise. If you have the lens mounted to a camera and the camera on its back in AF mode, the drive screw will help hold the lens elements in place while the focus ring turns. You can tighten one screw and check it before reassembly. When it works well enough for you, just tighten those three screws and reinstall the filter ring.

When I took the photo above, I noticed some dirt near the focus ring screws, and the somewhat exposed focus helix threads. Dirt can end up in there and cause uneven focus effort. So I removed the focus ring screws and pulled off the focus ring, bending the contact brush. It's not meant to come out that way, though that's how a manual focus lens comes apart. I had to take off the mount and aperture ring, three screws and separate the lens body, then reinstall the focus ring, bend the contact brush back and reinstall that, then put it all together. I took the oportunity to clean the lens and regrease the focus helix for better manual focus feel.

Last edited by Just1MoreDave; 10-21-2011 at 03:14 PM.
10-21-2011, 03:08 PM   #4
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Thanks for the great answer.

10-29-2011, 09:51 AM   #5
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Thanks so much Dave! I bought a set of JIS phillips head screwdrivers and I ended up using the 00 size for the filter ring screws and the 000 size for the focussing ring screws. It was exactly as you said to adjust, and took less than 10 minutes. Now it is fine on my K5, K7, and K 2000. You really made a difference here, so thank you!

07-16-2012, 05:42 AM   #6
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Thanks for the tip! I managed to solve my FA 50 f 1.7 not focusing on infinity problem!


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