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08-31-2011, 02:36 PM   #1
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Slack fstop selector ring on M 50mm F2

Hi guys, well my camera was delivered today, and considering the very cheap price I'm really happy with it, apart from a few cosmetic blemishes - which are to be expected - it seems in great condition. After having a quick play with it though one thing I did notice is there appears to be a slight problem with the lens.

The ring you turn to select the fstop seems to be very slack when going from 22 to 2, there's just no resistance at all. However when turning it the other way there is definite resistance, so much so that when setting the fstop to 4, as soon as I let go the ring jumps and ends up nearer to f16.

I assume that this is spring loaded and should infact 'click' into place at each setting. Does anyone know if this is an easy fix? Considering the price I paid I doubt it's worth taking it to a shop, but if it's easy enough to dismantle the lens to try and see if anything is obviously out of place or worn then I'm more than happy to get the screwdrivers out.

Any help/advice would be greatly received. The lens is a smc pentax -m 1:2 50mm.

Cheers

Dan

08-31-2011, 03:06 PM   #2
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Um, yes, there is definitely "a slight problem with the lens" as you say. I hope you didn't pay more than US$20 for it. Can you return it for a refund? I haven't disassembled any of my M50/2's yet so I can't help with repairs. Good luck!
08-31-2011, 04:21 PM - 1 Like   #3
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Hi... You didn't mention if the aperture seem to function or if you can see the blades. The single direction is worrisome... does it at least feel as if it still have a ball bearing when it clicks? I does sound a as if it was re-assembled wrong and commonly an unfamiliar person will loose track the ball bearing. The other thing is if the tab that operates the "full-aperture" is working correctly. A setting arm inside is commonly inserted backward and can make for problems. Also check to see the slot running along mount base. There should be a metal tab that mates to the camera to sync with the meter of the camera. If that or the other turns out missing, then it is not worth fixing and you should get a refund.
08-31-2011, 08:47 PM - 1 Like   #4
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My first guess is also that the ball bearing is missing. Here are some photos of the lens partially disassembled. The lens on the left in the first photo has just the mount removed. Normally these screws are tight but someone has already gotten them off once, so maybe that helps. A JIS screwdriver is best for these 5 mount screws. On the right, that lens has had a lot of parts removed. The photo is trying to show that the stamped plate slides right off.



In this photo, the lens on the left has only had the plate removed. On the right, I've removed the mechanism that moves the aperture blades. It has the aperture lever that sticks out of the back of the lens, a leve that sticks deeper into the lens to control the blades, a spring to shut the blades, and a stamped part that works with the other chromed stamped washer to set the blades at a certain opening for each stop on the aperture ring. The chromed stamped part connects the aperture ring to this mechanism with a small projection on one side. You don't need or want to remove the copper guide bar that is on the lens on the right.



This photo shows the aperture ring removed. The bearing is just above the orange diamond. It's trapped between the lens main body and the ring. A spring underneath it pushes it against the ring.The "teeth" on the ring make the ring click at each stop marked.



09-02-2011, 11:17 AM   #5
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Hi Guys, first of all I'm happy to report that I've managed to fix my lens so I'd like to say a massive thank you, especially to MysteryOnion who pinpointed the problem and Just1MoreDave - those photos were extremely useful and helped me 'see' problem and how to fix it.

Looks like the lens was taken apart at some point in it's history and when it was put back together they didn't fit the tiny ball bearing that sits between the body of the lens and the fstop selector ring. The one way resistance looks like it was purely coming from the washer/plate pushing against the aperture blade mechanism.

Thankfully I'm the type of person who keeps parts of things 'just in case they come in for something in the future' so after raiding my jar of ball bearings I was able to find a suitably sized one to fit ontop of the small spring. Re-assembled the lens and each fstop setting firmly clicks into place now, and stays there after I release my grip.

I only paid the equivalent of about 50usd for a ME super body, the lens mentioned above, a flash and camera bag and that included postage too, so I think trying to chase a refund for the lens would have been more effort than it was worth in this particular case, especially when you can get replacement lenses so cheaply on ebay, to be honest if I wasn't able to fix this myself then I'd have been more than happy to just throw it in my cupboard for spare parts and buy a replacement, but thanks to your advice I've been able to get it back fully functioning again.

Thanks once again guys, your help is much appreciated!

Cheers

Dan
09-02-2011, 11:39 AM   #6
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Hey, nice job repairing. Thanks for proving me right to the wife... got to keep those parts, never know when they come in handy.
09-03-2011, 01:09 PM   #7
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Cheers oh that sounds quite familiar - I keep trying to spread the idea that there is no such thing as 'clutter', there's only stuff that I need right now, and stuff that I might need in the future! My theory keeps seeming to fall on deaf ears though
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