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03-10-2012, 03:35 PM   #1
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SMC-M 1:2 "tight" aperture control

Hi everyone,
I've really searched and read all I could find here, but nothing looks like my problem.
I've got a 50mm 1:2 SMC-M lens with a stuck aperture ring. Sounds familiar? Well, it's far from it. I was of course suspecting some gunked up grease. I dismantled the whole lens just to find out, after thoroughly cleaning everything, that the aperture dial simply has a too tight fit on the lens barrel. You literally have to force it a little bit on the lens barrel - it's quite obvious that no grease will ever do the trick here.
Moreover, to make it even more weird, the inner aperture ring (the one directly linked to the external dial) also has a too tight fit inside the lens barrel. There's no way this whole ensemble is ever going to move. Not the way it is now.
I don't know the history of this lens at all - I got it together with a ME Super (blocked, of course) for an amount that would equal some 13 dollars. I'm far from being new to camera overhauling - I've taken apart all my film cameras down to the level of cleaning escapement cogs tooth by tooth and never seen anything similar.
The lens has obviously been opened before and put back together carelessly. Two clear signs: wrong position of the helicoid (infinity = 1,7 meters, heavy front focus) and the missing steel ball that gives you the aperture click stops.
Frankly, it looks like the aperture dial and ring do not belong to this lens barrel. I mean they really do seem to belong there, but just until the point when you put them together. The outer dial misses some 0,05mm on the inside diameter to match the lens barrel outer diameter > forced fit (56.95 vs 57.00). The aperture ring is 0,2mm wider than the inner diameter of the lens barrel (55.3 vs 55.1) > another hopeless situation. A piece of heavy furniture would move more freely on the floor than my aperture dial around the lens.
My guess is someone switched parts between lenses belonging to quite different production lots. I really cannot think of any other reasonable explanation.
I'd appreciate some feedback on this issue, if anyone ever encountered such a situation. The only way out seems to mechanically trim the two non-fitting elements


Last edited by danieldumanescu; 03-10-2012 at 03:49 PM. Reason: typos
03-10-2012, 04:47 PM   #2
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Your diagnoses sound solid, doubt that anyone can add much to it, I sure as heck can not.
Is it even worth messing with?.
03-10-2012, 05:45 PM   #3
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Indeed, that sounds rather strange. Why someone would even bother to switch aperture rings round is beyond me though ...

Keep it for spares and just buy another.
03-10-2012, 08:17 PM   #4
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You most likely have a "Frankenstein" lens, made of parts of several donors. M series 50 f2 changed from metal to plastic on some parts along its production. You may even have mixed parts from an A series.
Since it is not working at this point anyways, I'd suggest to take out a dremel tool or a file and go for it.

Thanks,

03-11-2012, 12:30 AM   #5
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Thanks everyone for your replies.
Ismael, you bet I'll "shave" those two pieces. I hate having non-working items. Since neither the outer dial nor the inner ring fit, my assumption is that the lens barrel is the "alien" here. I don't have an A series for reference, but I have the A series lens diagram (the one from pentax-manuals.com) and that one looks different. So it still has to be a M series barrel. But trimming it is much more difficult than trimming the other two components.
The next step will be compiling a nice working ME Super from two non working ones
03-11-2012, 03:24 PM   #6
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Well, I've got it working but this was no joyride.
I've grinded the outer dial and the inner command ring, to find out that the metal pin that couples the inner and outer rings was too short (no, I didn't grind that, it was too short from the very beginning. Ok, I thought, looks like copper, let's solder a tin supplement - it worked.
Next, I saw that the nuts connecting the two pins (the one going to the outer dial and the one connecting with the camera mount) to the ring did not pass properly under the screws of the helicoid guiding piece above. More grinding.
Since I didn't have any 2mm steel ball for the aperture click stops I filed half of a 3.2mm ball, enlarged the corresponding hole and fit it there. Works like a charm.
Eventually, got the aperture working perfectly.
Last surprise: focus doesn't get smooth no matter what I try. I've thorughly cleaned everything and tried two types of grease (not together, of course, there was one more cleaning involved in between). Useless, it still staggers. It's perfectly usable, but it's kind of annoying.
And... oh... I think I forgot to mention the fungus. Things wouldn't have been complete without it. But it came off just fine.
Bottom line, leaving alone the bumpy focus issue, the lens is back among the living. But it's been like a bad joke, I don't even feel like I've achieved anything. I don't know if there are fake Pentax lenses out there, but this one was either a fake or a Frankenstein. Or a Frankenstein fake.
I didn't get to test the glass yet though, I'll see what comes out eventually.
03-11-2012, 03:33 PM   #7
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Wow! After all that, hopefully it's at least fairly sharp
03-11-2012, 03:41 PM   #8
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QuoteOriginally posted by Den Quote
Wow! After all that, hopefully it's at least fairly sharp
Well, I don't have a Pentax DSLR (yet) to tell right away, so I'll have to wait until I meet I friend who has one. I doubt I'll have enough time to fix the ME Super body and run a film through it until then

03-12-2012, 05:03 PM   #9
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I would be hesitant to mount that on my camera, major source for all possible contamination.
Good luck.
03-12-2012, 10:18 PM   #10
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QuoteOriginally posted by Ex Finn. Quote
I would be hesitant to mount that on my camera, major source for all possible contamination.
Good luck.
Everything has been thoroughly washed washed in isopropyl alcohol, hydrogen peroxide and ammonium chloride. Of course, before relubing the mechanics
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