A C mount is a type of lens mount commonly found on 16mm movie cameras, closed-circuit television cameras, machine vision cameras and microscope phototubes.
C-mount lenses provide a male thread which mates with a female thread on the camera. The thread is nominally 1.000 inch (25.4 mm) in diameter, with 32 threads per inch. The flange focal distance is 17.526 millimetres (0.6900 in) for a C mount.
Merely to say that a lens is "C-mount" says very little about the lens' intended use. C-mount lenses have been made for many different formats. C-mount lenses are built for the 8mm and 16mm film formats and the 1/3", 1/2", 2/3", 1", and 4/3" video formats, which corresponds to a range of image circles from 5 to 22 mm diameter, approximately.
For example, for the 4/3 format, a 12-mm lens is a wide-angle lens and will have a retrofocus design. For the 2/3 inch format, a 12mm lens is "normal" and can have a simple and fast double Gauss layout. For the 1/3 inch format, a 12-mm lens is long and can have a telephoto design.
Some TV lenses lack provision to focus or vary the aperture, so may not operate properly with film cameras. Also, some TV lenses may have bits that protrude behind the mount far enough to interfere with the shutter or reflex finder mechanisms of a film camera. The user should determine the suitability of using C-Mount lenses for their application.
Although C-mount lenses have a back focal distance far too short to be used with 35mm film SLRs or any existing digital SLR, they can be mounted on interchangeable-lens mirrorless digital cameras. One such system is the Micro Four Thirds. However, the vast majority of C-mount lenses produce an image circle too small to effectively cover the entire micro-four-thirds sensor, with its approximately 22-mm diagonal. This produces what is popularly called vignetting but is better described as a mismatch between camera format and lens format.
In those cases where a C-Mount lenses has focus and f/stop controls, using these lenses requires manual focus and exposure. The adapter provided is simply a mechanical conversion.