Lens Terminology Focal Length Focal length is the distance between the camera sensor and the center of the lens.  The greater the focal length, the larger the image will appear.  Therefore, the greater the focal length, the more the lens becomes telephoto in application.   F# / F-Stop The f/number is an indication of the brightness of the lens. It is the measurement of the ratio between the focal length and the diameter of the entrance pupil (where the light enters the lens). The f/number is directly proportional to the focal length and inversely proportional to the effective diameter of the lens. It determines the amount of light reaching the camera sensor.  The smaller the value, the larger the opening and the brighter the image produced by the lens.   Minimum Object Distance The Minimum Object Distance indicates how close the lens can be placed to the object for shooting. It is measured from the vertex of the front glass of the lens. Lens Mount 1) Any camera may use a lens equal or greater than the format of the sensor.   For example, a 1/2" format camera may use a 2/3" format lens. 2) F-mount lenses are compatible with C-mount cameras.  An F-To-C mount converter is required to use F-mount lenses with C-mount cameras, see lens accessories. 3) F-mount lenses are compatible with CS-mount cameras.  A 5mm extension tube (C-To-CS mount converter) and F-To-C mount converter are required to use F-mount lenses with C-mount cameras, see lens accessories.