I have no trouble with c-mount lenses designed for 16mm film cameras, but plenty of trouble getting infinity focus with c-mount lenses designed for surveillance cameras.

@Alex I don't understand what you mean by "flange back", but with infinity focus problems it usually goes like this: As you screw your C-Mount lens into the adapter which is already mounted on the M43 camera, set the lens to infinity and switch the camera on. Watch the viewfinder screen as the lens goes deeper into the adapter; if the infinity focus gets better but doesn't quite get there, You have to use some means to get it closer to the sensor - either a slimmer adapter or, (more likely) you'll have to very carefully grind the rear of the lens barrel until you get infinity focus. If it focuses through infinity, all you need to to is make up some shims so that the lens sits further forward from the sensor while it's screwed tight. If your lens is expensive, you might settle for no-infinity-focus rather than modify the lens. Or maybe it's in focus at a smaller aperture.

@Alex .." if the adapter assures the proper distance between lens and sensor, there should be no problem with focusing".

The adapter is made independently of the types of lens fitted to it. For example, most of my C-mount lenses are off my Bolex or Pathe 16mm cameras, where the film plane is closer to the lens mount thread. The M43 sensor is further back and the adapter cannot be made thin enough to compensate, nor set back with a longer lip - the camera's hole won't allow it. For info on film planes, see: http://www.giangrandi.ch/optics/lenses/focalcalc.html When it comes to getting the lens's numbers where you can see & reach them, many lenses will allow you to turn the focus-ring around and reset it. (Especially easy with Pentax 6mm CCTV lenses). This is done during collimation, also matching the numbers to the distance of the subject from the camera, once the infinity focus is achieved. I'd advise you to take a cheap lens, (maybe scratched), some duct tape, leather gloves, a grinding wheel, maybe a Dremel... protect the lens, protect yourself, clean metal dust away from lens and camera - but have some fun!

Before buying a particular adapter, it's simple to calculate how thin the rear flange will need to be: Carefully- carefully!! -insert the lens into the m43 camera body mount, without an adapter. (How you achieve this without risking touching lens glass to sensor is up to you, your tools & ingenuity). Buy some digital callipers. Perhaps you could use auto feeler guages to measure your max rear flange thickness. Turn camera on and manipulate lens forward & back until you get infinity focus. Use improvised shims to measure the gap behind lens barrel- that is how thin your adapter will have to be in order to do it without any machining. You may be disappointed; rarely will you achieve infinity focus without lens-grinding. Also, some adapters are of thick alloy. Many C-Mount lenses require an adapter as little as 1mm thick - otherwise the extra grinding required to bring the lens closer to the sensor would destroy the lens. The adapter has to hold the lens on, too. It must be strong. So forget machining deep into an alloy adapter. The best thin ones are of high tensile steel. Adapter sellers sometimes show measurements in their eBay listings. If not, ask.

Come to think of it, I'm using the exact same adapter as @disneytoy

@kujina

For an adapter you need skinny, strong steel :-)

You still won't get any adapter which will avoid the grinding at the end of barrel. A skinny one may limit grinding the sides.

Once you have the adapter, get the lens cap on the lens, cover glass at rear, duct tape everywhere so no brass dust gets inside, leather gloves on, eye protection, fine grinding wheel. Grind that lens until it fits into the adapter and focuses a little beyond infinity, then make shims using blister-pack plastic and emery paper, sand them down until you get just infinity focus.

A lot of cleaning-up and re-wrapping lens required between trial-and-error. Go slowly. digital callipers will minimise having to try on camera.

Don't over-grind rear of lens; it can get so thin it'll fall apart or break apart while you're shooting. Do it bit by bit and this won't happen. Happy grinding!

Thanks for the responses @Roberto & @meotai what is a good/better adapter to use, I think I remember reading that some have less space.

@Roberto

I think I've might of bought the same adapter as you have pictured?

When I screw the 6mm 1.2 lens on the adapter (I haven't started filling yet) it barely screws on because the lens is too wide so it sits on the front face of the adapter, is this what happened with yours? Or on a better suited adapter the lens fits into the adapter's circular recess a little bit?

I just want to make sure I won’t need to excessively file the lens because the adapter isn't right for it.