CCTV-C/CS Mount Cameras
The advantage to a cctv 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. Cctv 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.
Many times, the best option is to use a varifocal lens. This will allow you to vary the focus from 5 to 50mm. So when you install the camera you can fine tune the focal distance and angle of view. An example of where you might use a varifocal lens is outside (or inside) a large commercial building where you need more distance than 35 ft. and the varifocal lens will allow you to adjust the focal distance to your preference.