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C-Mount Question

Discussion in 'Adapted Lenses' started by jrworthy, Jun 5, 2012.

  1. jrworthy

    jrworthy Mu-43 Rookie

    Jun 5, 2012
    Washington DC
    I recently picked up a couple of Pentax CCTV c-mount lenses but I am having problems with focusing beyond just a few inches from the camera.

    I have a OM-D and an E-PL1 and both have the same results. My c-mount adapter is from fotodiox.

    While I understand the vignetting I don't understand why I am having difficulty achieving focus on objects that are farther away than 5 inches or less from my camera. I have removed the focus screws in both lenses so I can move the focus ring.

    Does anyone have any pointers?
  2. f6cvalkyrie

    f6cvalkyrie Mu-43 All-Pro

    Feb 12, 2010
    Brussels, Belgium
    Maybe the adapter is too thick ?
    I had problems with adapters from Fotodiox, but those were for Nikon lenses.

    I hope this helps,
  3. ean10775

    ean10775 Mu-43 All-Pro

    Jan 31, 2011
    Cleveland, Ohio
    I'm just speculating, but it sounds like a problem with your adapter. As I understand it, the various c-mount adapters vary in their flange distances. It may be the case that yours mounts the lens either too far from the sensor or too near, thus affecting the focus distance.
  4. jrworthy

    jrworthy Mu-43 Rookie

    Jun 5, 2012
    Washington DC
    Do you have any c-mount recommendations?
  5. lecturedoodle

    lecturedoodle Mu-43 Rookie

    Nov 7, 2011
    Is it possible that there's a macro ring screwed on the back of the lens or on the Fotodiox adapter?
    This is a picture of one my c-mount lenses, with the macro ring to the right.
  6. ean10775

    ean10775 Mu-43 All-Pro

    Jan 31, 2011
    Cleveland, Ohio
    That was my other thought, but I don't believe that any of the Pentax lenses come with macro rings.
  7. jrworthy

    jrworthy Mu-43 Rookie

    Jun 5, 2012
    Washington DC
    you are correct, there are no macro rings on either the lens or the adapter.

    I have a feeling that I will have to hunt for the correct adapter. It will probably be frustrating but I have little choice.

    Aside from fotodiox are there any adapter rings that you can recommend for c-mount lenses?
  8. OPSSam

    OPSSam Mu-43 Regular

    Dec 18, 2010
    Is it possible it is a CS lens you are using. It has the same mount but a different (shorter by about 5mm according to Wikipedia) normal mounting distance.

    I have a normal c mount (and a 1940s era one no less) that has no such problems with the Fotodiox mount.
  9. winx14

    winx14 Mu-43 Regular

    Dec 1, 2010
    Brooklyn, NY
    I bought my adapter from RainbowImaging for under $10. It's fine except it doesn't focus to infinity (need to turn the focus back a little bit to do it).
  10. Brianetta

    Brianetta Mu-43 Veteran

    Sep 5, 2010
    North East England
    Brian Ronald
    I have a 4mm CS lens (and a CS network camera). I tried it on my C adapter on my E-P1, but the only things I could get in focus were actually in contact with the front element. Extreme!

    The macro rings are, I found to my delight, simply C to CS adapters. Using one, I put my C mount 25mm lens on my network camera, and it became a seriously long telephoto. With a further CS adapter, it might find use as a close-up ant-farm-cam.

    Dragging myself back to jrworthy's subject at hand, I'd suspect that the lens might be wider than the recess in the adapter.

    The flange distance on a C mount is about two and a half millimetres shorter than micro four thirds, so the lens has to be mounted slightly back, within the body of the camera. To this end, the adapter has a recess. If part of the back part of the lens is wider than this recess, it's going to get up to two and a half millimetres of close focus, probably rendering it only able to focus out to a few inches.
  11. RobWatson

    RobWatson Mu-43 Hall of Famer

    I have some lenses that do not actually screw all the way into the adapter as the body is too 'fat'. Even c-mount cameras often (like always) have the incorrect back focus distance which is why one should never buy one unless the back focus is adjustable.

    Many of the lenses are also adjustable if you are adventurous.

    5" inches is pretty severe ... seems like the back focus is off by 1mm or so - which is a lot.

    I have a metabones adapter that does not quite work but the Fotodiox works fine.
  12. jrworthy

    jrworthy Mu-43 Rookie

    Jun 5, 2012
    Washington DC
    Definitely using a C-Mount lens. I have a new adapter on order so hopefully it will work better.
  13. jrworthy

    jrworthy Mu-43 Rookie

    Jun 5, 2012
    Washington DC
    I think this might be the issue. It seems as though the lenses that I have are not able to screw into the full depth of the threads on the mount.

    Hmmm, what to do now? I guess I will have to return the lenses.
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