Lomography Fisheye 2 + Leica M mount lens cap = focus free Fisheye lens in M mount.
Wide lenses are easy to modify for M-mount because they require no rangefinder coupling, and often no focus at all, so I bought a Fisheye 2 with a view to modifying it. (Un)fortunately after a year, it yet to die on me. I find it wasteful to hack a working camera, so I was delighted to find a second one with a broken shutter trigger in a bargain bin for $7. I measured the flange-focal length and found it to be OK for conversion to M-mount, so I dismantled and broke apart the camera till all that was left was the lens. Then all I did was cut a hole in a lens cap and file down the outside of the barrel till it focused right when put together. I used the same Versachem Plastic Welder epoxy glue that I usually use to stick the barrel and lens cap together. Altogether, it was a quick one hour’s project for me.
My only concern was the Fisheye 2 has a curved film plane, but it didn’t seem to make much difference when shot on a camera with a flat film plane or sensor.
According to information on the web, the lens is supposed to have an aperture of f/8 but I found that it actually meters around f/16. Anyway, at least now I can adjust shutter speed and ISO to expose properly. I have tried it on two cameras so far, a full-frame Sony A7R and a Ricoh GXR + A12 module with an APS-C sized sensor. I find the APS-C results uninteresting, but the A7R produces the same nearly full circle fisheye pictures that the Fisheye 2 is known for.