This is the ratio of the focal length to the effective diameter of the lens (f/A). It is not a measure of the efficiency or the transmission value of the lens. The smaller the f number the more light is passed.
In a lens, the ratio of the focal length to its diameter. The smaller the F stop, the more light is passed. The F stop also effects the depth of field.
Light Factor (f)”. The ability of a camera lens to pass light. A value used to indicate the speed of a lens where the smaller the number the better the lens “a fast lens”, the normal f stop in CCTV lenses is f/1.4 or f/1.2. Each increase in f-stop decreases the amount of light passed through the lens by 50%. The normal f-stops are: f/1.0 – f/1.4 – f.