BS ISO 517:2008

1   Scope This International Standard pertains to apertures and related properties of photographic lenses affecting the illuminance at the centre of the image. This International Standard specifies aperture markings for all types of lenses used in still cameras, and gives tolerances for the stop numbers . It also defines aperture stop , entrance pupil , focal length, relative aperture and stop numbers , and gives methods for their measurement or determination. This International Standard applies only to lenses focused on objects at infinity; that is, at least 50 times the focal length of the lens.