The aperture setting on a lens. The number is the focal length of the lens divided by the diameter of the aperture. As a result, larger f-stop numbers represent narrower aperture sizes. F-stop numbers are used so that exposure settings for a particular scene can be expressed without having to know the focal length of the lens used. The f-stop number is the size of the lens’s maximum aperture, measured as a fraction of the focal length of the lens. On some zoom lenses there may be two apertures quoted f/4-5.6, for example. This means that the maximum aperture gets narrower as the lens is zoomed in. The maximum aperture on the lens barrel may also be expressed as a ratio, such as 1:4-5.6.