According to Wikipedia, 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, designated as “1-32 UN 2A” in the ANSI B1.1 standard for unified screw threads. The flange focal distance is 17.526 millimeters (0.6900 in) for a C mount.
There is a similar lens mount called CS-mount. CS-mount lens has a flange focal distance of 12.50 millimeters (0.492 in), but is otherwise the same as C-mount. Many cameras can accept either type of lens, but it is important to make sure that camera and lens are compatible and set up properly. For example, if you try to use a CS-mount lens on a C-mount camera, since the focal length of the lens is too short, the image formed by the lens cannot be focused on the camera, therefore the picture obtained on the camera will be out of focus. On the other hand, if you try to use a C-mount lens on a CS-mount camera, you can move the lens further away from the camera by 5mm, and the image formed by the lens can still be focused on the camera to get a clear picture. The way to move the lens away by 5mm is to add a 5mm C-mount extension tube between the C-mount lens and the CS-mount camera. This 5mm extension tube is often called C-CS mount adapter.
As shown in above picture, C-mount lens can match C-mount camera, the same is true for CS-mount lens and CS-mount camera. To match C-Mount lens and CS-mount camera, a 5mm C-mount extension tube is needed. Unfortunately, there is no way to match a CS-mount lens and a C-mount camera.
In some applications, you may want to focus on some objects at a distance shorter than the designed object distance of the lens. As we know, in an imaging system, when the object distance decreases, the image distance will increase, so you need to increase the distance between the lens and the camera sensor by adding an extension tube or spacer in between, as shown below:
Even for adjustable focal length lenses, you may want to use extension tube to coarse adjust the focal length and then fine tune with the lens’ adjustability. This will largely increase the range of the focal length that can be adjusted by the lens.