Friday, February 5, 2010

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.
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7 comments:

  1. Ray DengFebruary 6, 2010 at 6:28 PM

    can you modify mine? or how do you do this with a file? i use the 12.5mm on an e-p1

    ReplyDelete
  2. Ben KrasnowFebruary 8, 2010 at 9:43 AM

    Ray, I'd be too worried about breaking your lens or adapter, so I don't want to try machining your parts. I already broke one adapter that I bought earlier. I suppose it would be possible to do this job with a file, but it will take a long time. The idea is to remove the least amount of material from the lens and the adapter. I chose to cut a 45* angle on both the adapter and lens. This allows a tiny amount of the adapter's metal to fit between the camera body and the lens. If you hold the lens up to the camera without the adapter in place, you can see how little space there is available. Good luck! -Ben

    ReplyDelete
  3. Ray DengFebruary 9, 2010 at 1:10 PM

    hey ben,

    i just adapted it today! i just bought a normal file from the local hardware store and it was relatively easy.

    i haven't tried it yet, but will let you know the results.

    just wanted to say that your blog is rather inspirational! you've taken the DIY spirit to a new level.

    best,
    ray

    ReplyDelete
  4. Ben KrasnowFebruary 9, 2010 at 4:39 PM

    Great! Thanks for the comment. Good luck with your new lens. -Ben

    ReplyDelete
  5. AnonymousAugust 16, 2011 at 1:59 PM

    Thanks for posting this info, interested in how your mod worked for you ray, were you able to test it yet?

    ReplyDelete
  6. Gan EdenJune 30, 2013 at 4:09 AM

    There needs to be some kind of adapter that does not have that 37mm wide recess or a wider recess to enable the flange of the C mount lens to screw closer.
    Please join our group here and suggest ideas please.

    www.facebook.com/groups/cmountm43

    ReplyDelete
  7. Eelke aka ToonJuly 19, 2014 at 4:46 AM

    How much millimeter would you say, have you removed?

    ReplyDelete