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02-11-2010, 05:10 PM   #1
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M series prime stuck aperture

I just ordered a pair of M series primes from KEH that were listed in the as-is section with stuck apertures. They are the 50mm f2 and the 35mm f2.8. Does anybody have instructions on how to take the "back half" of these lenses apart? Better yet, has anyone dealt with this problem before?

Edit: Will I need a lens wrench to fix these?

02-12-2010, 03:26 AM   #2
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Wow, how much did you pay? The 50mm f2 is a very common lens and usually pretty cheap even in excellent condition. (I had one I paid $20 for)

As to how much work to fix, depends on why the apertures are stuck. The lenses I've seen with stuck apertures are usually because of lubricant that has seeped out onto the leaves and almost glued them together. A fix means getting that gunk off. That gunk is usually on both sides which means taking the lens completely apart to get to all surfaces with some kind of cleaner. I tried it with one junker lens I had just to see what was involved and it didn't seem like a lot of fun or especially worthwhile IMO unless it was a very special lens.
02-12-2010, 12:39 PM   #3
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The 35mm was a whole whopping $3. I would have just skipped the 50mm, but I figured it would be a good $0.89 learning tool or paper weight!
02-12-2010, 12:50 PM   #4
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QuoteOriginally posted by Steve Beswick Quote
The 35mm was a whole whopping $3. I would have just skipped the 50mm, but I figured it would be a good $0.89 learning tool or paper weight!
Wow, I have to look there sometime.

I have photos of an M50/2 disassembly but they are hard to get to at the moment. Mostly just take screws out until you see the problem.

02-12-2010, 04:10 PM   #5
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As a general rule, I've found that if you can get to the front or back side of the aperture mechanics a little lighter fluid (or equivalent) and patience work wonders if the problem is simply gummy oil on the blades.

A Q-tip (a small cotton swab) saturated with a solvent, or a hypodermic needle with same, works well - the key ingredient is patience. Let the solvent work.

And if you don't open both sides, don't saturate the aperture blades as fluid will dribble down on the un-opened optic elements then you'll have to go all the way.

H2
02-13-2010, 10:13 PM   #6
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This is one of my first disassemblies. I had two M50/2s so I used one as a reference and took apart the other. The one on the left has just had its mount removed. On the right, I've taken off a lot of the front, and from the back, I've removed the mount, the aperture ring and the stamped steel plate that connects the aperture ring to the aperture mechanism.


Here is the central part of the aperture mechanism out of the right lens. This part has the aperture lever that sticks out the back of the lens, a lever that sticks further into the lens and moves the blades, a spring, and other stuff that's hard to describe. This part could be your problem but oiled blades are more likely to be the issue. By the way, don't remove the two copper-colored things like I did; not necessary and will just lead to trouble.

Here I show how the steel plate goes onto the aperture ring. In the background, you can see the place where the 2mm bearing would go, below the orange diamond. It's pinched between the ring and a spring, and bumps against the grooves on the inside of the ring. The spring will launch the bearing if you're not careful when you slide the ring off. It's not necessary to remove the aperture ring for your repair. You should be able to get a decent grip on the rear lens group and unscrew it here. That will give you access to the rear of the blades.

This is the front disassembly if you need it. UYnscrew the "name" trim ring and take off the filter ring (not shown). The brass washer is holding in the front lens group. Remove that and you have access to the other side of the blades. You can clean the blades in place as Pacerr suggests above. You ought to be able to get the individual blades out from this point, which makes cleaning easier, but it's trickier to reinstall and adjust them.
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