Cctv By Afex Fire & Safety

C-mount lenses provide a male thread which mates with a female thread on the camera. The thread is nominally 1 inch 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 mm (0.69 inches) for a C-mount, 12.52 mm for the otherwise identical CS-mount.
The advantage to a C-Mount Camera is that the lens can be changed. If you need to see further than 35 or 40 ft then you'll need a c-mount camera with a special lens. C-Mount lenses are available from 4mm to 50mm. A 4mm lens provides facial detail and a 70 degree angle of view focused up to about 35 ft. An example of where you might use a fixed 4mm lens is in a small office, or at home to focus on your driveway.
A higher millimeter lens will provide further distance, but narrows the field of view. So,a 16mm lens would provide about a 15 to 20 degree angle of view at a focus distance of about 35 ft (or a 70 degree angle of view at about 140 feet). In general, a 8mm lens is like a 4mm lens zoomed in 2 times. Similarily, a 16mm lens is like the 4mm lens zoomed in 4 times.