C-Mount flange distance

M

mbelassie

New member
Hi there,

I couldn't find a thread about this particular problem. I just received a lovely Fujinon-TV 9mm f1.4, and I'm aware that most lenses like this need their mounts filed to achieve infinity focus.

What's confusing me is: Even when I hold the lens over the sensor of my GH1, as deep as it will go, so that that rim of the lens is against the silver GH1 mount, I still don't get anything other than macro focus. Does that mean that sanding a few mm off the end of the mount, so that it sits deeper in the adapter, won't make any difference? I may be completely missing the point, but don't want to mangle the lens if there's no physical way it's going to get close enough to the sensor.

I'd be really grateful for any advice you guys can give.
Marcus
 
i recon the distance is around 18mm - similar to sony nex. get a m4/3 to cmount adaptor and try it. you might need to machine the adaptor a little bit. and as a last resort, as you suggest, make mods to the lens.
 
C-mount lenses have to sit physically inside the m43 body. M43 is about 20mm, c-mount is about 17.5mm. So if the rear of the lens is too big to fit a few mm into the lens port, then yeah, forget that lens, the modification might be more hassle than it's worth.
 
Barry_Green said:
C-mount lenses have to sit physically inside the m43 body. M43 is about 20mm, c-mount is about 17.5mm. So if the rear of the lens is too big to fit a few mm into the lens port, then yeah, forget that lens, the modification might be more hassle than it's worth.
Click to expand...

Thanks for the advice guys! Thought I'd give an update. I think the flange distance must be measured from the back of the main body of the lens, not the protrusion beyond the screw thread.

I was filing the back of the lens, and the whole rear piece screwed off. It could then sit close enough to the sensor. In fact, screwed all the way into the adapter it was about a mm too close. I unscrewed it to the correct distance and put a mark on the adapter and the lens that I could line up again later. Unscrewed the lens, put some superglue on the thread and quickly screwed it in until the marks matched up. Mounted the lens, works beautifully :D

Its a 9mm, so equivalent 18mm on the GH1, and 36mm with 2x digital zoom to get rid of vignets. However, the silver protrusion extends so far into the camera there's a good chance it's blocking the shutter. Thus haven't tried it for stills, but video's what I wanted it for.

For other people trying to attach their c-mount lenses, this is the Fujinon 9mm f1.4 TV lens. There's a good chance their other lenses in this range work in a similar way; it's stiff at first, but the rear part of the lens - the bit that stops it fitting into the adapter - can be unscrewed. I'd be happy to help anyone who wants more advice - the above may or may not be well-explained!
 
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