C-Mount is a camera / lens standard that is commonly found on CCD/CMOS Digital Cameras as well as 16mm film cameras. The C-mount specification has several dimensions associated with it:
Mounting Thread = 1 inch x 32 tpi UN 2A
Length of Lens Thread = 3.8mm
Lens Flange Focal Distance = 17.52mm
Camera Depth (C-Mount Face to Image) = 17.52mm
The above dimensions are employed by all C-Mount lens manufacturers as well as all C-Mount camera manufacturers in order for any lens and camera combination to image properly.