The function of the lens is to collect light reflected from the scene and focus it into an image onto the CCTV camera sensor.
Most CCTV applications use a fixed-focal-length (FFL) lens, which, like your eye's lens, covers a constant angular field of view (FOV). That is, the FFL images a scene with constant magnification.
A large variety of CCTV camera lenses are available with different focal lengths (FLs), which will provide different FOVs. Wide-angle, medium-angle, or narrow-angle lenses produce different magnifications and FOVs.
How to use this tool
This handy FOV calculator will help you in determining which lens is right for your application. Enter the camera lens format, the distance to the object, focal length of the lens, and click "Calculate" to see the field of view you can expect with that lens. It also works in reverse. If you know the field of view you need, enter it and the distance to the object along with camera format to find the focal length you will need to get the field of view desired.
How to read license plates
Typically you need a horizontal field of view of 10 feet or less to be able to read a license plate. Thus, at 100 feet with a 1/3" standard camera, you will need a 50 mm lens to get a horizontal FOV of under 10 feet. This doubles to a 100 mm lens at a distance of 200 feet, and so on.