It can be used as a holder between the focal length and magnification (placing the intermediate ring between the C mount camera and the C-mount lens will increase the focal length and increase the magnification), or as a holder between mechanisms.
This intermediate ring is C-mount standard, one side is female and the other is male ring, and the C mount can be extended.
This ring is usually known as "close-up ring" and the working distance will be shortened. Also, please note that the longer the darker the longer it becomes.
It is also convenient when making digital microscope devices using C-mount tubes.
C-Mount Standards:
Inside diameter 25.4mm (1"), pitch 0.794mm (32 teeth / 1"), flange back 17.526mm ,
Used in security camcorders, CCD, CMOS cameras and other optical equipment for video observation.