Cosmicar-Pentax 8-48mm on an eos m c-mount focus question

williamandrews · 06-24-2020, 06:31 AM

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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Thagomizer · 06-24-2020, 11:29 AM

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.

williamandrews · 06-24-2020, 12:54 PM

Wow. ok - well that pretty much nails it. thank you! That's very kind

Thagomizer · 06-24-2020, 04:52 PM

Originally posted by williamandrews

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.)
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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.

Vicent · 12-18-2022, 08:41 AM

Hi. Did you find a deeper adapter? I have the same focus problem with a Cosmicar 12.5mm 1.9 in my Canon eos m.

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06-24-2020, 11:29 AM	#2
Thagomizer Pentaxian Join Date: Jun 2014 Location: London Ontario Posts: 2,068	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
williamandrews New Member Join Date: Jun 2020 Posts: 2 Original Poster	Wow. ok - well that pretty much nails it. thank you! That's very kind

06-24-2020, 04:52 PM	#4
Thagomizer Pentaxian Join Date: Jun 2014 Location: London Ontario Posts: 2,068	Originally posted by williamandrews 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.) Ù 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.

12-18-2022, 08:41 AM	#5
Vicent New Member Join Date: Dec 2022 Posts: 1	Hi. Did you find a deeper adapter? I have the same focus problem with a Cosmicar 12.5mm 1.9 in my Canon eos m.