I used a Vixen 0.6x C-mount reducer. It was made for 2/3" CCD cameras but it actually covers the whole 4/3 sensor! Funny enough, it never actually worked with my C-mount lenses. The rear fit into a C-mount just fine, but the front (female part) was made for 1.25" eyepieces. It might be good for astronomy and telescoping, but pretty useless for general photography out of the box. It was expensive too, something like $185 used if I remember correctly. I know there are cheaper reducers out there, but I bought this one thinking it will work with C-mount lenses, as the name implies. I was wrong. Because of micro 4/3's flange focal distance, this reducer will only work with Nikon lenses, and even longer flange focal mounts like T-mount, and PL-mount. By the way, this particular reducer has been discontinued now and is almost impossible to find online.
I unscrewed the actual reducer from its original mount and put it through a regular C-mount to micro 4/3 adapter that I got off Ebay. I filed the adapter's opening to allow for the reducer's larger diameter. Then, I had to file the front and back "hoods" of the reducer to get the optic closer to the sensor in the back, and closer to the lens in the front. It is all held in place by superglue. When mounted, it's a very tight fit; I really can't get the reducer any closer to the sensor without hitting something or getting it jammed.
Here is how it looked like modified to c-mount :
And this is how the finished adapter looks, with the Nikon mount:
It fits my E-P1 like a glove to a hand! I calibrated it for infinity focus with Nikon lenses (actually, it goes past infinity, but only slightly). There is no blur, no tilt/shift effects, no stray light pollution, or any other outside factors affecting the image quality. Lens speed is increased, and it's pretty sharp for what it is, although contrast is decreased. Here's a test... it was shot at ISO800 to avoid blur from my handshake, but I'll have some low-ISO shots soon for you guys:
ROKINON FISHEYE 8/3.5 NO REDUCER:
OLYMPUS E-P1, F3.5, P MODE, -1.7 EV, ISO 800, 1/13 sec
ROKINON FISHEYE 8/3.5 WITH 0.6x REDUCER:
Being a fisheye, the distortion is (obviously) caused by the lens and NOT the reducer. The vignetting seen above is also not from the reducer but from the lens. The fisheye lens is DX format, made for Nikon's APS-C, so it creates a smaller circle than a lens designed for a full-frame would. I actually shaved off the fisheye's hood to get more coverage for this test! I get no vignette with other, full-frame 35mm, lenses.
Here is a vignette test for this fisheye with the reducer. This tests the lens vignette, NOT the reducer's:
http://img822.imageshack.us/img822/5453/ep1reducerfisheyevignet.jpg
And here is a test with the same lens on full-frame (Canon 5D) and APS-H (Canon 1D) cameras:
http://www.360pano.de/en/samyang_5D_1D.html
As you can see, the lens coverage with the 0.6x reducer is somewhere between APS-C and Canon's APS-H. This means that some of Nikon's DX lenses will vignette with this reducer.
I have a couple of more Nikon lenses to test the reducer with: Nikkor 105/2.5, Nikkor Sigma 30/1.4, Nikkor 43-86mm F3.5, and Nikkor 28/3.5. I will provide more sample images soon :happy: