What have I done? PK Sigma APO 400mm 5.6

Discussion in 'Adapted Lenses' started by Darlar, Apr 23, 2013.

  1. Darlar

    Darlar Mu-43 Rookie

    Jan 20, 2011
    Seattle, WA
    I bought this lens on the cheap online. Upon arrival I noticed that the aperture was not working. I have zero familiarity with PK mount lenses so I fiddled and fiddled and apparently made the situation worse, as the click aperture ring became click less. Disassembly was a logical next step - I said I bought this on the cheap, right?

    In the course of fixing whatever was wrong (I can't tell if something in the iris engagement assembly was hitched, or if the blades themselves were too gummy to open (they look super gummy)) but after more fiddling, with the lens mount disassembled, I was able to engage the iris to open and close either by manually flicking the little tab in there, or when I put the aperture ring back onto its tab by moving it open/close.

    Ok, then I saw the very tiny ball bearing that accounted for the click and figured that I had bent its housing a little too far down and thus my click less aperture. Fixed that.

    Finally, I saw the restraining post for the iris and noticed that while it was inserted in a spot to hold the lens at F22 there was room to move it back and close the lens down even further.

    So, and I said on the cheap already right?, I unscrewed the post and moved it back.

    Here's my question: did I do what i think i did and allow this lens to stop down even further? Is there any way to figure out what stop it might be at? The iris definitely closes down tighter, so I think I'm in the right ballpark.

    The picture shows the original travel that went from 5.6 to 22.

    I know just enough to be dangerously stupid; can any of you think of any repercussions to the lens for doing this?

    On reassembly, I noticed that a wide metal ring with a tab was left out. The lens works fine, and I figure that particular ring had something integral to do with the Pentax shutter action and zero to do with me using the lens on my Em5.

    Attached Files:

  2. Cruzan80

    Cruzan80 Mu-43 All-Pro

    Aug 23, 2012
    Denver, Co
    Sean Rastsmith
    Not sure what damage you could have done by allowing the blades to go even tighter. Possibly it has extra holes to allow fine-tuning of the blades. It will still open up all the way, correct? Aka, you just let the throw become wider, instead of leaving the amount of opening/closing the same, and just moved the end points?

    IIRC, the only thing that the metal ring with a tab may affect (for you) is resale. Many of the older lenses have a tab-like device, to allow the lens to stay wide open when focusing, and only close down when firing (or by pressing a specific button). My guess is that you disabled that when you removed the ring. As the m43 adapter doesn't have this function, you didn't lose anything for this camera.
  3. Darlar

    Darlar Mu-43 Rookie

    Jan 20, 2011
    Seattle, WA
    Yes, it has just increased the throw and not shifted it.

    That's a good idea, and I'll check again and see if your idea is the case. I'm not worried about resale - Its really not worth that effort. But if I were to sell it, it would be with the m4/3 adapter and hopefully to another adapted lens user.
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