Adapting Special C-MOunt Lenses

greeches

greeches

Veteran
I have many c-mount lenses that can use a generic adapter to attach to a m43 camera. The focus works perfectly. I have two lenses that I'd like to adapt to the GH2, which must have a different spacing in terms of flange distance.

Here is one (18-108mm f2.5):

http://www.bestscientificweb.com/index.asp?site=products&order=Brands&productid=28&menuid=Qioptiq

Here is the Other (8-48mm f1.2):

http://reytecimaging.com/computar-h6z0812.aspx

Any leads would be great. I'm not opposed to taking them apart.
 
One very easy way is to use a C-mount extender. The downsides are that you lose 1-2 stops of light, a variable sharpness decrease, and (with 1/2" extenders) you can get slight corner vignetting when stopped down.

The other option is to machine (or dremel, grind, file, etc) the rear lens collar until it fits within the c-mount adapter, and then shim as needed.
 
I removed the bottom ring and seem to allow for closer setting of the lens. The lens focuses properly now!

P1010744 (Medium).jpgP1010745 (Medium).jpgP1010746 (Medium).jpg

The black "tube" had a small ledge on the top where the screws went. This prevented it from screwing into the c-mount adapter. Once removed, it tightened right up and can focus properly.

Open at 1.2 is a bit soft, but many c-mounts in Tele-cine mode exemplify this. It vignettes severly without tele-cine mode.

Now to the 18-108!


I would avoid optical correction at all costs, as I got the lens for its speed.
 
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Good job. Some c mount zooms are more complicated than others, definitely more complex than primes. Metal lenses are easier because you can machine them down as opposed to plastic ones.
And yes the hidden risk with these fast lenses is that they're generally soft wide open. I successfully modified a Canon 18-108 before, but it was a f1.6. Alternatively, you may also consider machining a relevant c mount mount adapter - ones with deep flanges.
My most prized lens to date is the Cosmicar/Pentax 8-48 f1.0, very sharp wide open... Working on another one right now actually
 
Awaiting that very lens via eBay, Ed. Going to try it with a 1.5x or 2x extender... If it works as with my Tamron 4-12, I'll lose a full stop (2 with the 2x), but won't have to use ETC... I think avoiding etc noise at high ISO will more than compensate for the light loss. If it fails, Ed's test of the C/P 8-48/1 has already convinced me it'll be useful.

Useable 12-72/f1.4, nearly parfocal... Makes me slightly giddy - if it works.
 
mcbob said:
Awaiting that very lens via eBay, Ed. Going to try it with a 1.5x or 2x extender... If it works as with my Tamron 4-12, I'll lose a full stop (2 with the 2x), but won't have to use ETC... I think avoiding etc noise at high ISO will more than compensate for the light loss. If it fails, Ed's test of the C/P 8-48/1 has already convinced me it'll be useful.

Useable 12-72/f1.4, nearly parfocal... Makes me slightly giddy - if it works.
Click to expand...

Ah you got the 4-12mm Tamron? Just got that a while back and did the whole sanding down thing. So the 2x extender doesn't need ETC? Great news - so about 1.8 ish from 1.2. Let me know if you have clips online
 
I tried a 2x extender with it.. Still vignettes, now covers about 90%. Though mine was aleady modified so I can't focus properly now... I used a computar 2x brand
 
Sure Gary... it was actually your video that nudged me into buying it. - http://vimeo.com/27131652

Clips with the extender are the sandbox shots and the stormy skies and wind. A few caveats - the extender prevents the lens from opening up wider than ~6mm. Also, it actually does vignette slightly at the corners, but only when the aperture is stopped down about 1/3 of the way.

A 2x extender will eat up about 2 stops of light (so that 1.2 becomes more like 2.2-2.4), and a 1.5x will lose about one stop.
 
Should be fine. There is a small aluminum screw piece. This piece holds the lens just a little too far from the sensor to focus properly. I need to have it machined (taking 1-2mm out of it) so it will sit closer to the sensor. Relaly nothing dangerous about it, as the screw piece is removable from the lens entirely.

Will update w/ pics whenever I get time...
 
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