C-Mount cameras and lenses were the first used in CCTV applications. The lens screws directly into the camera providing a distance of 17.5mm between the back of the lens and the imager. Current CS lenses are smaller and better suited for smaller cameras. The lens to imager distance is 12.5mm. A 5mm spacer called a C-ring, or C-mount is used with CS lenses to bring the lens to imager distance to 17.5mm. C-Mount/CS-Mount are lens types with different flange distances. They determine compatibility with camera bodies.