A confession: I've glued the Cosmicar 50/1.9 into a PK body cap. I could probably rig it so the rear element sits about 15mm closer to the sensor, and still clear the mirror. That *might* allow focus out to 1-2m. Maybe. I've dealt with a less extreme issue, fitting a lens meant for the Argus C3 (The Brick!) with ~35mm register so that it focuses to infinity on my K20D. Pushing its rear element back 12mm puts it in the right position.
But putting a C-mount lens in the same place still leaves a ~15mm gap in registers. Hmm, that's close to Leica LTM register of ~29mm. So I'll grab a Steinheil 135/4.5 and put it on a 2mm-thick M39-PK adapter and throw it on a camera and see where it close-focuses. The answer is... just over 1m. Now I'll skip the adapter and just handhold the lens in close, but not so close as to foul the mirror. And the answer is... maybe out to 1.5m.
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SO WHAT'S POSSIBLE? If you can handle the lens, just stick it into your dSLR's mount and see how close it will focus. If it's reasonable, here's a cheap rig:
Get an eBay PK body cap, a thick black one, not the thin white guys Pentax supplies. Cut a hole in the middle. Stick the lens in. The lens base should extend NO MORE THAN 11mm from the base of the body cap. The mirror is set back about 13mm and you want to leave a little clearance. When you've positioned the lens correctly, use contact cement to glue it in place. Contact cement is easily dissolved so the lens is recoverable.
DOF on a 25mm lens when stopped-down might allow focus out to 2m or more. The perspective of a portrait shot at 2m... isn't too terrible. No huge noses, not with a lens that wide. I just measured -- at 2m distance, subject height in portrait (vertical) mode on an APS-C cam is about 1m, say a half- or 3/4-body shot.
So it wouldn't be an impossible project. How cheap is the lens?
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Post #5 by RioRico
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