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04-16-2012, 12:58 AM   #1
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Problems with lens, thoughts...

Hello. My m 50 1.4 wont focus to infinity and if u shake it it tinkles slightly. It's not the end of the world i paid only 50 bones, but it would be nice for walking around if it was fixed. Regardless, portraits and what not are still super sharp. I wouldnt mind tearing into it if i could fix it. Thoughts?

04-17-2012, 12:07 PM   #2
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You could probably open it up and take a look, but be aware that it has a lot of parts and very small screws. Be sure you have the right tools for it, and the patience and organization to deal with very small parts and to remember what goes where and what goes can be confusing...I've opened up a couple of my M50's usually to tighten up things, and in one case to try and replace the knurled focusing ring. (None of the others would fit) And just to take a look, since I have one that's not usable any more. The most troublesome part is the retaining ring for the objective lens. It's easy to slip and damage the ring if you're not careful and don't have tools to do it with.

I don't know what might be rattling around, probably a screw that came loose. I've had a couple of them do that. One hung in the aperture ring and caused it to lock up. My A series 50mm has a rear element that screws in, it came completely loose one evening while shooting sunsets, I freaked out until I found out it simply screwed back in by hand.

First, check all the external screws with a small screwdriver, there is a chance it might be just the back mounting plate loose or something similar.

If I have to open it up, I usually mark each lens with an arrow using a Sharpie marker when I take it out, so I know which direction it faces when reassembling. Get one backwards and it's difficult to get it right again since you don't know which one is wrong, unless you mark each one. Also be careful about dust. You don't want a ceiling fan blowing on high speed while working on a lens. And DO NOT use a screwdriver that does not fit the screw. You can get inexpensive sets of small (jewelers) screwdrivers that should work, I've always kept a couple of sets on hand because I work on small things a lot. But then I have training in optical instrument repair, computer repair, small engine repair, (you should see how small the carb on a chainsaw is) and other similar jobs and hobbies.

SO...If you have the patience, mechanical ability, tools, and some knowledge of small repair jobs in general, it might be possible but be careful. It's very easy to lose parts, especially the tiny screws and detent balls that make the "click" in the aperture ring. I lost one a while back...but it was in a BB gun. They are usually spring loaded, if it gets away that spring shoots it halfway to Minnesota...The one in my 135mm took me about 5 tries to get it back in place. And a magnet to find it in the carpet...(fortunately it never went far or it would have never been found either) Drop a screw, same thing, better have a magnet handy. I also have an older junk lens I can use for spare screws...that helps a lot.

Oh...check the retainer ring on the back of the rear lens. If that's loose, chances are it will lose infinity focus and probably will rattle around too. If you don't have a tool for it you can have someone hold the lens while you GENTLY use two small screwdrivers to snug it up.
04-18-2012, 03:37 PM   #3
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Original Poster
Thanks for the reply. I was thinking of trying to buy a parts 50 and try to see how i do taking that apart first. I'm mechanichally inclined but have rather large man mitts. I could see it being challenging.
04-18-2012, 04:13 PM   #4
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Found this link which might help you out. You can look and see what is involved and then decide for yourself if it's something you want to try.

Pentax 50mm repair 1


04-20-2012, 08:39 PM   #5
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May have been said above, but the set screws for the focus fine adjustment are coming loose (one is out already apparently) so your fine adjustment is drifting, probably every time you turn it against one stop or the other. I could tell you how to get at them on the identical A lens but not the M.

On the A you pop the front trim ring off and then you can see 3 tiny screws (JIS though they look like phillips they are not) to set fine adjustment you loosen them all and then while holding the outer focus right against the infinity stop, put the screwdriver into one of the screws and turn the whole assembly that the screws go into without letting the focus ring turn (like using a rotary phone with a pencil sorta ) either right or left to correct the focus at infinity then tighten the screws. Basically the screws hold the inside of the lens with the glass tight to the outside focus ring that you turn which has the stops on it, so its slipped and the outer part is hitting the stop before the inside reaches the infinity end of its rotation. I would avoid locktite though some suggest it, these things are tiny and super easy to snap off and with locktite on if you ever need to take them off again they will break. Have to just use finger tips tightening them actually and be super delicate. Also get JIS screwdriver set off amazon or something.

EDIT: just looked at that link, its the part in step 3 I believe though these things are definitely different inside. Not sure what he is talking about on his step 11, but it sounds more technically correct than the "looks correct" way I did it.

Last edited by PPPPPP42; 04-20-2012 at 08:57 PM.

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