Originally posted by Na Horuk 
Is the focus ring bent? If you inspect it carefully, it might be a little oval which causes it to jam at some specific focus distance. Mine did that after it was dropped. I sent it to a repair store that fixed it and charged as much as I would otherwise spend on getting another copy.. even now it rubs the barrel a little at some distance.
That's pretty easy to diagnose. Look at the front of the lens. There should be a consistent small gap between the front barrel/ring with the filter threads and the focus ring. The focus ring turns and the filter ring doesn't, so see if that gap changes at different focus positions. Can a filter still screw into the filter threads? That mean the filter ring is still round or very close to it. With some patience, the focus ring can be squeezed back into shape if it got slightly squashed.
If nothing obvious is bent or out of shape, dirt is a likely suspect. You'd have to disassemble the front of the lens to clean it. The ring engraved with "SMC Pentax 1:1.7 50mm" unscrews via the filter threads. Some kind of friction tool is used to press against it and turn. Then six screws are revealed. The ones with arrows can be removed to take off the filter ring:
Then you'll see this:
The three screws marked with red arrows here hold the focus ring on. You may find some dirt to clean out here and if you're lucky, that fixes it. But you'll probably have to remove this ring as well. That means a bit of extra work resetting infinity focus later, not a huge issue. If you take off the focus ring the lens will look like this:
The aluminum ring turns and focuses the lens. It has a set of fine pitched outer threads and steeply pitched coarse inner threads. The outer threads move a little and the inner threads move the optics in or out a lot. At infinity, the aluminum ring looks like the photo, screwed down almost as far as it'll go into the lens body. Your lens will probably show some exposed coarse threads on the optical tube. Use an old toothbrush to sweep out any crud here.
I can't figure out any other problem that would crop up in normal use besides dirt or being squashed out of shape. I suppose you may end up having to remove the optical tube to entirely clean the lens. That's a major project, covered pretty well here:
https://www.pentaxforums.com/forums/maintenance-repair-articles/217082-takuma...egreasing.html.
For tools, you'll need a friction thing to get the trim ring off in front. Anything rubber that fits into the filter threads and doesn't touch the front glass is OK. Sink stoppers, chair leg protectors, plumbing parts, whatever. The screws are JIS, technically a whole different type of screw than Phillips. You can usually get by with a Phillips #00 screwdriver on these screws because they aren't terribly tight. I'm not sure what's available in the UK.
Last edited by Just1MoreDave; 04-24-2013 at 08:55 PM.
Post #7 by pezmaker
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