C-Mount is an internationally recognized standard that is followed by all manufacturers of C-Mount lenses. Here’s the full definition for those who are new to them or may have doubts.
A C-Mount lens has a screw thread with a diameter of 1 inch (25.4mm) and 32 TPI (threads per inch). The “flange focal distance” is 17.52mm (0.69 inches). This distance is measured from the lens mounting plane (i.e. the rear surface of the lens which butts up against the camera) to the focal plane (i.e. the plane of the film or sensor). The C-Mount standard also defines the thread length as being 3.8mm.
The C-Mount standard does not define the coverage or projected image circle of the lens. This can differ widely: C-Mount lenses may be designed for 1/2″, 2/3″, 16mm, 1″ or other formats. It should also not be confused with the CS-Mount standard, which has the same thread mount but a smaller flange focal distance of 12.52mm.
As we see, the C-mount standard does not define the width of the flange itself. As some Micro Four Thirds and Sony NEX users have found out, wider lenses may not be able to obtain infinity focus when mounting them with an adapter. Although both the Micro 4/3 and NEX formats have very short flange focal distances compared to other DSLRs, they are still slightly longer than the C-Mount one. NEX has a flange focal distance of 18mm, Micro 4/3 of 20mm. When using C-Mount lenses on these cameras this means that the lens must actually be set nearer to the sensor than the flange plane of the camera. The adapter must allow them to be recessed slightly inside the camera’s mount opening, but this obviously means that the flange width cannot be wider than this opening. Exact measurements vary with the make of the adapter and the geometry of the back of the lens can also make a difference, but as a general rule the flange diameter of lens cannot be wider than 37.2mm for use at infinity focus on Micro 4/3, or wider than 44mm on NEX. Some lenses can be modified to fit, others will not be able to obtain infinity focus but can still be used for closer focusing or macro work.
C-Mount to Micro Four Thirds Adapter
Actual configurations may vary depending on manufacturer and model. Some have two or more “steps” instead of one. There are also “ultra thin” models that may help obtain infinity focus on certain lenses.
C-Mount to Sony NEX Adapter