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12-18-2013, 02:29 AM   #1
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Pentax-M 50mm f/2 Rebuild Issues

Sorry if this isn't the right place, but I didn't see a more suitable section. I have taken apart this lens to clean mold off the inside of the front and rear elements, and all went perfectly well. I put it all back together how it came apart, and everything still seems fine. That is until I mount it to the camera. While the aperture lever moves perfectly smooth off camera, when mounted, it won't stop down past maybe f/4. I can't for the life of me figure out what is going on. There is no way I could have put the aperture assembly on wrong, as it only fits one way, since the screws are spaced unevenly. I know there is no way to tell what is wrong without seeing the lens, but does anyway have experience with having this lens apart or have had trouble like this? Thanks in advance.

After looking more, I'm even more baffled. It almost seems as if the mount itself is keeping the aperture arm from moving the last little 1-2mm to fully open to f/2. I can turn the mount to where the screw holes are off by maybe a screw width, and it works fine. I guess I'll have to take it all back apart... sad, it was going so well.

12-18-2013, 03:04 AM   #2
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Actually, it looks like an issue with the aperture arm inside the camera. What exactly does this little arm do? Is it supposed to keep the aperture fully open until the shutter is released or something along those lines? The light meter still works accurately with the aperture ring as far as I can tell... I'm just a little confused. Thanks
12-18-2013, 03:09 AM   #3
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It's not something I ever thought about or looked into, but I'm guessing that is the case, that the aperture stays fully open until the shutter is released, so that you can see through the lens via viewfinder.
12-18-2013, 03:26 AM   #4
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With the lens unmounted the aperture should be wherever it is set on the ring. When you move the lever the aperture opens all the way and when you release it the aperture should quickly snap to wherever the ring is set. If this is not happening, it is a lens problem.

12-18-2013, 09:59 AM   #5
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QuoteOriginally posted by elliott Quote
With the lens unmounted the aperture should be wherever it is set on the ring. When you move the lever the aperture opens all the way and when you release it the aperture should quickly snap to wherever the ring is set. If this is not happening, it is a lens problem.
That is what it does. I guess I just didn't realize initially how the camera and lens worked together. I'm sure it's fine. Thanks for the input.

I did apparently have something slightly out of spec, as it now only stops to 2.8 instead of 2.0. The aperture assembly mounts to the body with three screws in slotted holes to allow for adjustment. Apparently I had it off by a tiny fraction of a mm, just enough to stop that last little click of the ring. No loss, as I typically don't shoot it at f/2 anyway, I find it a little too soft for my liking. Everything else works as well as it did before I tampered with it. Still silky smooth and solid feeling.
12-18-2013, 10:59 AM   #6
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QuoteOriginally posted by Pentakumar50 Quote
The aperture assembly mounts to the body with three screws in slotted holes to allow for adjustment...
There are different models of M50/2, basically the expensive build and the cost-cutting build. This sounds like the cost-cutting build, which looks like this when you take it apart from the front.



I think you are describing the screws outside the aperture blades in a silver colored ring. Those would probably work. I took apart two lenses but I don't have that particular version to test the theory.

The older build makes this kind of reassembly mistake easy. It looks like this after you remove the trim ring and filter ring. The screws marked with red arrows hold the whole optical assembly - front group, rear group and aperture - into the lens body.



This is the optical assembly, just because I have a photo:



Anyway, when you go to reassemble the lens, the assembly goes in, the metal bar marked with a red arrow goes into a fork on the body, and it looks fine. But you can rotate the assembly in place with the screws loose and see the blades move. You have to set the aperture ring to f2 and rotate the assembly until the blades just disappear. Then tighten the screws. Or you can set the aperture ring to f22 and rotate the assembly until the blades close fully. Tighten the screws right there, then move the aperture ring to f2 to see if they disappear at f2.
12-18-2013, 11:34 AM   #7
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Thanks! Yeah, this is apparently the cost cutting build, and yes that silver ring is the one I was referring to. I tried to put it back on how it came off, but there really weren't any clear marks on the ring to match up with the screw heads. I saw some minor smudge marks, but nothing precise. It's close enough anyway. If I ever miss f/2, I'll take it back apart and fix it, but for now it works at 2.8, so that is good enough for me. Have the focus dialed back in too. Though I am tempted to take it back apart, since I apparently let too much of the grease get wiped off of the plastic threads on the barrel of the focus assembly. It still focuses smoothly, but every so often I can hear a very muffled low frequency squeak, like you hear with dry tight tolerance mechanics. Any idea what grease would work best for a lens like this? I've never had to apply more grease to a lens before. I wonder if a 50/50 dry graphite and grease mix would be good? Graphite is pretty amazing at making things like this quiet and smooth.
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