Adapting a Cosmicar 12.5mm f/1.4 c-mount lens to the GH1

The micro four thirds (and standard four thirds) camera format makes it possible to use old c-mount lenses with decent results. The lenses with shorter focal lengths will not cover the image sensor fully, but it seems that anything 25mm or longer will have full sensor coverage. These lenses are very cheap (eg $30), and the quality is surprisingly good.

This is the cosmicar 12.5mm f/1.4. I got it here:
http://www.surplusshed.com/pages/item/l10128.html

One problem is that the fast, short focal length lenses have a very wide barrel. In fact, it is too wide to fit into the micro four thirds -> c-mount adapter. I machined both the adapter and the lens so that they would fit together and also achieve infinity focus.

The lens has a brass mounting plate with the C-mount threads on it. It comes off very easily by removing the three screws around the periphery.


I used a lathe to cut an angle onto the edge of the brass mounting plate.

I cut the same angle onto the C-mount adapter. I bought mine from an eBay seller called rainbowimaging.


I'm pretty sure infinity focus has been achieved. If you think this image looks soft, it may just be the overall characteristic of the lens. It is $30.

The real benefit of theses lenses is that they are very fast (f/1.4) and wide, so they are ideal for indoor video. The 16:9 format works a little better with the image circle produced by the lens.

This has been very helpful in choosing lenses:
http://spreadsheets.google.com/pub?key=p9kkgjwEQQQ-HJwvNDobeEw


Also note that this is my second attempt in adapting the cosmicar to the camera. I had bought a c-mount adapter previously and machined it without machining the lens. I eventually machined away so much material (accidentally) that the adapter became too weak and broke immediately. I then realized that some material must be removed from both the lens and the adapter to make it work.