ƒ-number/ƒ-stop - The standard way of describing the Relative aperture of a lens. Calculated as the lens Focal length, divided by its Effective aperture. It makes it possible to compare the aperture settings of different lenses. The standard series - ƒ1, ƒ1.4, ƒ2, ƒ2.8 and so on, is in steps of a factor of two; each higher number represents half the amount of light being passed.
