1pc C-MOUNT 5mm Extension Tube Adapter C-CS Mount Adaptor Spacer Ring For CCTV Lens Item Specification: Material: Metal, Aluminum alloy. This is an adaptor mount which allows the use of C-Mount lenses on to a CS-mount CCTV camera body. This adaptor ring is used in conjunction with specific C-Mount ring adaptors for various mirrorless camera bodies. The distance from the reference surface to the focal point of the C-mount is 17.562 mm, and the focal length of the CS is 12.5 mm. If you are installing a C mount lens to a CS mount camera, you will need a lens converter. This kind of converter is called C-CS ring.