[Kompe]

Hi there!
Forgive me if this thread should be put in the SLR lens forum instead. I'm sure some advent-friendly moderators can move it if that is the case


As the title explains, I have trouble stopping down to f/22 with my newly acquired, manual marvel. I can stop down to f/16 just fine, but the aperture blades will not move the slightest when turning the ring to f/22. Every sound and movement in the mechanism elsewhere seem all right. At first I thought that the difference from f/16 to f/22 might be so subtle that I wouldn't notice, but after looking closely, absolutely nothing happens. Every step up to f/16 work flawlessly.

The aperture blades don't seem oily, but they are not as snappy as my SMC Pentax-M 50 f/1.4. They are 'softer' and more quiet in their movement, and not as instantaneous as the 50 mm (which is very quick) - but they are instant enough. I don't know if this is how it should be. It looks more like there is something mechanical somewhere that blocks the blades from moving, and not just grease.

I unscrewed the back K-mount plate (or what you would call it), just to see if there was something odd, but I couldn't find anything. I haven't taken a lens apart before, and I am therefore a bit reluctant to do so (even though I'm quite used to pick apart lots of other mechanical stuff).

Do you experts know what could be the issue, and how difficult it is to fix?


I plan on mostly using this lens with my old P30n for portraits, so I will probably never use f/22, but it's just good to know that my lens work as it should in every way.

Thanks!

[Just1MoreDave]

The Pentax-M 50/1.7 can end up like that. All of the lens elements and aperture blades are in one structure. It is only connected to the rest of the body by a chrome bar that moves the aperture blades, which slides into a fork on the body. The structure can rotate maybe 10 degrees when you slide it into place. Three screws keep it in the body and keep it from rotating. Those screws are one of the first things removed. When the lens is reassembled, it's easy to forget to rotate the structure so that the blades are fully open when the aperture ring shows f1.7, and fully closed at f22. Someone could just tighten the screws so that the blades only close to f16. The lens would work fine otherwise. The fix is not that hard - take the "name" ring off, three screws hold on the barrel with the filter threads, the other three screws hold the structure in place. Loosen those and rotate the structure until you find a place where the blades are fully open when the aperture ring is at f1.7 and fully closed when the aperture ring is at f22.

The big question after all that is, how is the M85/2 put together? I don't know if it's the same. If it has the same structure with all the elements and aperture blades together, it probably has the same rotation adjustment. I found this video in the lens disassembly database, and I couldn't tell. The video does show the person has unscrewed the focus helix, which I would avoid. It's not necessary and it'll take you an hour to put it back together.

You Tube

You Tube

[Kozlok]

If you’re not going to use f22, I’d leave it alone.

[Not a Number]

Sounds like a cleaning is in order. Blades can be oily at the ends where you can't see it. Or someone put it back together without adjusting the diaphragm position correctly.

[Kompe]

Thanks a lot for a thorough and helpful answer, Just1MoreDave!
I know much more about the problem now. I'll think a bit more about whether I want to fix it or not, as it doesn't seem to be the simplest task out there. It will probably stay as it is for now, and maybe I'll deal with it later.

[ZombieArmy]

In all honesty you will never, ever use f22 on that lens. If the aperture is snappy otherwise I wouldn't worry about it.