How Different is between C-Mount Lens and CS-Mount Lens?

Lens can be divided into C-Mount Lens and CS-Mount Lens.

C-Mount Lens

The flange back (distance from the surface where the lens comes in contact with the camera to the focal point) of C-Mount lens is 17.526mm (0.69 inch). This is the only difference between the C-Mount lens and CS-Mount lens. C-Mount lens, as well as CS-Mount, is 1 inch diameter with 32 TPI (threads per inch). A C-Ring (5mm spacer ring) is required when a C-Mount lens is to be mounted to a CS-Mount camera unless the camera has built-in Flange-back Adjusting Ring.

CS-Mount Lens

The flange back of CS-Mount lens is 12.5mm (0.492 inch). This is 5mm shorter than the C-Mount lens and you need a 5mm spacer ring (or, C-ring) when you mount a C-Mount lens to a CS-Mount camera. (So, CS-Mount camera is always compatible with C- or CS-Mount lens. But, C-Mount camera is not compatible with CS-Mount lens. Most professional cameras are CS-Mount cameras and include a C-ring.)