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06-24-2020, 06:31 AM   #1
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Cosmicar-Pentax 8-48mm on an eos m c-mount focus question

Hello!

So i shoot video on an adapted EOS m. I really like c-mount lenses and saw this cosmicar/pentax tv zoom 8-48mm f1, but when i mounted it - i got the sever vignette i was expecting - but i can't focus it? I tried a different adapter that brought the back element closer to the cmos, and i did improve - but, moving focus ring didn't make a huge difference to the (blurry) image at all. Is it broken do you think? Or is it simply not compatible...

thank you in advance!

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06-24-2020, 11:29 AM   #2
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Checking out the Wikipedia entry for" Flange focal distance", it shows C mount lenses registration distance is 17.526mm; for EOS EF-M, 18mm. Your C-mount lens is not going to focus to infinity unless you can mount the lens so it is .474mm inside the body mount plane. Any adapter that introduces additional distance between the camera and lens will act as a macro extension tube and preclude anything but close focusing. C-mount lenses will work on anything with a shorter flange distance than 17.526mm, like Pentax Q, or Nikon Z, as long as there's a suitable adapter that will allow mounting it at the appropriate distance for the given camera body.
06-24-2020, 12:54 PM   #3
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Wow. ok - well that pretty much nails it. thank you! That's very kind
06-24-2020, 04:52 PM   #4
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QuoteOriginally posted by williamandrews Quote
Wow. ok - well that pretty much nails it. thank you! That's very kind
You're welcome. I first learned about flange focal distances when I started putting off-brand lenses on my Canon 20D some years ago, learning that Pentax would work, but Minolta would not (at least not without corrective optics, which I wished to avoid.)

If you're into macro at all, reversing that Cosmicar will likely give you interesting results. I've been very pleased with shots I've managed to get with a reversed D-mount movie lens.

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