ISO INTERNATIONAL STANDARD 517 Third edition 2008-03-15 Photography Apertures and related properties pertaining to photographic lenses Designations and measurements Photographie Ouvertures et grandeurs associees relatives aux objectifs photographiques Designations et mesurages Reference number ISO 517:2008(E) @ ISO 2008 ISO 517:2008(E) Foreword IsO (the International Organization for Standardization) is a worldwide federation of national standards bodies (IsO member bodies). The work of preparing International Standards is normally carried out through IsO InternationalElectrotechnicalCommission(lEC)onallmattersofelectrotechnicalstandardization. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. IsO shall not be held responsible for identifying any or all such patent rights. ISO 517 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 1, Fundamental standards. This third edition cancels and replaces the second edition (IsO 517:1996) which has undergone minor technicalrevision. @ ISO 2008 - All rights reserved ili INTERNATIONAL STANDARD ISO 517:2008(E) Photography Apertures and related properties pertaining to photographic lenses Designations and measurements 1 Scope This International Standard pertains to apertures and related properties of photographic lenses affecting the illuminance at the centre of the image. ThisInternationalStandardspecifiesaperturemarkingsforalltypesoflensesusedinstill cameras,andgives 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. 2 Terms and definitions For the purposes of this document, the following terms and definitions apply. 2.1 aperture stop physical stop that limits the cross-section of the light beam that can pass through the lens to reach the centre of the on-axis image 2.2 entrance pupil image of the aperture stop as viewed from a point in the object space on its optical axis (the image of the aperture stop formed by the front elements of the lens) 2.3 exit pupil image of the aperture stop formed by the rear elements of the lens) 2.4 focal length of the photographic lens limiting value of the image size h' of a sharp imaged far-distant object h divided by its angular extension in theobject spacei.e - lim h' f : →0 tan(o) See Figure 1. 2.5 relative aperture of a photographic lens twice the numerical aperture where the numerical aperture is the sine of the semi-angle subtended by the exit pupil at the focal plane NOTE For photographic applications, the relative aperture is equivalent (within a 1/3 stop) to the ratio of the diameter of the entrance pupil to the focal length. @ ISO 2008 - All rights reserved 1