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11-08-2011, 02:58 PM   #1
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How to? C mount > K mount

Hi all (and mainly RioRico )

How do I mount C mount lens on my K mount bodies and retain some sort of focusing capability?
I couldn't care less for infinity but up to 7meters would be nice as the lens would be subsequently used for portraits etc.

Thanks for advices


11-08-2011, 03:16 PM - 1 Like   #2
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Forget it. Yes, it can be done. C-mount is 25mm thread and 17.526mm register. 25-42mm step rings (discs actually) are cheap and available. Just add a cheap safe wide-flange M42-PK adapter to mount on your dSLR. But because of the register difference (Pentax is 45.46mm) your focus range will be closer to 7cm, not 7m. I have tried this, with two Pentax Cosmicar C-mount lenses. The 50/1.9 lets me focus at about 3.5cm; the 12.5mm f/1.8 was totally impossible so I sold it.

C-mount glass will work for portraits on Sony NEX (18mm register) and maybe m4/3 (20mm register), and for anything on the Q (9.2mm register). But unless you're shooting portraits of spiders, a C-mount on your PK dSLR just won't make it. Sorry!
11-08-2011, 03:36 PM   #3
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oh well, and I got so excited about Scneider Xenon 25/1.4...
11-08-2011, 03:45 PM   #4
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Aside from the mechanics of the mount to get any reasonable focus would back the lens into the mirror. Remember you give up 30 mm in retested distance to get close to infinity.

11-08-2011, 05:02 PM - 1 Like   #5
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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.

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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