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05-29-2010, 01:06 PM   #1
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Pentax-M 50mm f/1.7 disassembly

Could anyone here point me to instructions for disassembling a Pentax-M 50mm f/1.7 please?

The problem with the lens is slight fungus in the rear elements (between the lens mount and the aperture blades).

I have disassembled one from the front but it's the rear this time. Basically, I'm asking can you go in from the rear of the lens? Any particular problems or best ways to go about it?

All and any help gratefully received.

Thanks in advance.

05-29-2010, 01:44 PM   #2
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It's easier than the front. If you can get the 5 chrome mount screws out, I would remove those and the mount. You'll end up with this:



Then you can get a decent grip on the rear lens group and unscrew it without tools. Be careful of two things: there's a loose black washer thing here, and the aperture ring can slide up and off. It's not a big deal as long as you don't lose the bearing when it does. The bearing is just below the orange diamond, trapped between the aperture ring and lens body. Sometimes the aperture ring can use a cleaning and regreasing (very little grease needed) if you want.

The rear group has three lens elements, but two are cemented together. Usually fungus doesn't intrude into the group. You'd need a lens spanner or substitute tool to remove the inner ring if the fungus is inside the group. Optical diagram here:

http://kmp.bdimitrov.de/lenses/primes/_optics/50f1.7.gif

With a lens spanner you don't have to remove the mount. The rear group has spanner slots that you can reach without much trouble. It depends on your tools and if the mount screws are stubborn.
05-29-2010, 02:31 PM   #3
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Thank you and much appreciated, Just1MoreDave. By the look of the lens, it's the inside of the rearmost element that has the problem.

As to the aperture ring, I usually tape these loosely to save chasing little ball bearings.

With the help I've got tonight, it should be OK.

As always, the forum has come up trumps. Thanks again.
05-29-2010, 03:03 PM   #4
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Just a small additional trick - hopefully not too late: use those one-way latex gloves while working (the unpowdered variant) and change them as soon as they become greasy.

05-29-2010, 03:09 PM   #5
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No, not too late. Won't be doing it until tomorrow. Thanks.
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