C-mount is a thread mount (1” in diameter with 32 threads per inch). This mount is commonly used on 8mm & 16mm movie cameras, CCTV cameras, microscopes and in industrial image processing.

Especially the lenses made for video cameras are optically superb and made by manufacturers like Schneider-Kreuznach, Angenieux, Taylor & Hobson, Kern, Canon, Fujfilm etc. C-mount lenses can be mounted on mirrorless cameras with an adapter. Typically cameras have much larger sensors than the lenses were designed for and lenses will vignette on these cameras. As long as the image circle is big enough, the resolution of most C-mount lenses is good enough for digital cameras.