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01-18-2010, 01:00 PM   #1
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135mm 3.5 with no aperture control

I am going to try to run some of 1970's manual Pentax SMC lenses on a micro four thirds camera. These are lenses that I have had for years with no use (I shot a Nikon DSLR before this) and a couple of them should work well with the mag factor so I figure it is worth trying.

Anyway one of the lenses and probably the most useful of those that I have is a 135mm 3.5 SMC. It was my grandfathers and has been sitting for probably 20 years. It looks to be unused but appears to have a mechanical problem in that the aperture is stuck closed. (at f22 give or take) The little lever that interfaces with the depth of focus button on the body does not work as it should and only moves a small distance.

I know these are not particularly valuable lenses so I am not going to send it off to be repaired. The question is does anyone know of a directions as to how one should take this lens apart?

I suspect that the linkage for the depth of focus is bent or perhaps dirty blades but I really don't know. I am not scared of small mechanical things and like a challenge but before I just start taking stuff apart I figured I would look for some more information.


01-18-2010, 02:11 PM   #2
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This lens, right?

Pentax Lens Review and Specification Database - 135mm F3.5

I don't have one of those any more. Pentax seemed to enjoy redesigning the linkage for every new lens, so I can't tell you exactly that you'll find. First, the way it's supposed to work is a two part process. The aperture ring sets the shooting aperture. Off the camera, the blades should move to that selection. The lever on the back of the mount then shifts the blades between wide open for focusing and stopped down for shooting.

I would take off the mount first, the five chrome screws. Sometimes these can be trouble - tight or easily stripped - so use a good screwdriver and technique. I think at this point you can remove the rear elements, which might give more access to the rest of the linkage. The elements (I think) come out in a group when you unscrew the light shield around them.

I can't remember anything else specific about this lens's linkage. One possible failure that might create your symptoms is if the blades have rusted together. It sounds worse than it is; they can often be unstuck with only some cosmetic damage. I think this is one of the Pentax lenses where the front barrel (in front of the focus ring) conveniently unscrews. That would be really useful if you need to get to the aperture blades.
01-18-2010, 03:35 PM   #3
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Thanks for the tips, I will pop the back off and see what I see.

Now I just have to find my small screwdriver set.
02-03-2010, 05:11 PM   #4
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I removed the back and manually moved the aperture actuator lever around a bit. Now it works great. I received my MFT adapter today and the lens seems to work great. I even played around with a doubler which makes it a 400mm equivalent on my GF1.



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