Entrance pupil 3166565 211722278 2008-05-11T20:25:36Z BenFrantzDale 41799 usually virtual, remove [[image]] link, which doesn't help. {{For|the part of the [[eye]]|Pupil}} [[Image:Aperures.jpg|thumb|A camera lens adjusted for large and small [[aperture stop|aperture]]. The entrance pupil is the image of the physical aperture, as seen through the front of the lens. The size and location may differ from those of the physical aperture, due to magnification by the lens.]] [[Image:030608 Pupil.jpg|thumb|The apparent location of the anatomical [[pupil]] of a human eye (black circle) is the eye's entrance pupil. The outside world appears to be seen from the point at the center of the entrance pupil. (The pupil itself, which lens designers would call the [[aperture]], is in a slightly different location because the image is magnified by the [[cornea]].)]] In an [[optics|optical]] system, the '''entrance pupil''' is a virtual aperture that defines the area at the entrance of the system that can accept light. Rays that pass through the pupil are able to enter the optical system and pass through it to the exit (neglecting [[vignetting]]). The entrance pupil is the image (usually [[virtual image|virtual]]) of the [[aperture stop]] in the optics that come before it. In a [[camera]], the aperture stop is the [[diaphragm (optics)|diaphragm aperture]] in the camera that the photographer adjusts to control how much light reaches the film. The setting of the aperture is typically represented by the [[f-number]], which is the ratio of the [[focal length]] of the lens to the diameter of the entrance pupil (not the diameter of the aperture itself). The entrance pupil of the [[eye]], which is not quite the same as the physical [[pupil]], is typically about 4&nbsp;mm in diameter. It can range from 2&nbsp;mm ({{f/|8.3|link=yes}}) in a very brightly lit place to 8&nbsp;mm ({{f/|2.1}}) in the dark.<ref>{{cite book | first=Eugene|last=Hecht|year=1987|title=Optics|edition=2nd ed.|publisher=Addison Wesley|id=ISBN 0-201-11609-X}}</ref> Depending on the lens design, the entrance pupil may be located within the lens system, in front of it, or even at infinity in the case of [[telecentric system]]s. The location of the entrance pupil is important in [[panoramic photography]], because the camera must be rotated about the centre of the entrance pupil to prevent [[parallax]] error when photographs are [[Image_stitching|stitched]] together into a panorama.<ref>{{cite web | author=Kerr, Douglas A. | year=2005 | title=The Proper Pivot Point for Panoramic Photography | format=PDF | work=The Pumpkin | url=http://doug.kerr.home.att.net/pumpkin/Pivot_Point.pdf | accessdate=2007-01-14}}</ref> <ref>{{cite web | author=van Walree, Paul | title=Misconceptions in photographic optics | url=http://www.vanwalree.com/optics/misconceptions.html#m6 | accessdate=2007-01-14}} Item #6.</ref> As a result, the centre of the pupil is sometimes called the "no-parallax point" of the lens.<ref name="Littlefield">{{cite paper|title=Theory of the “No-Parallax” Point in Panorama Photography | url=http://www.janrik.net/PanoPostings/NoParallaxPoint/TheoryOfTheNoParallaxPoint.pdf | author=Littlefield, Rik | version=ver. 1.0 | date=2006-02-06 | format=pdf | accessdate=2007-01-14}}</ref> ==See also== *[[Exit pupil]] *[[Pupil magnification]] ==References== <references/> ==External links== * [http://www.outline.be/quicktime/tuto The Grid] — Alain Hamblenne's method for a precise location of the entrance pupil on a DSLR camera [[Category:Geometrical optics]] [[Category:Science of photography]] [[cs:Vstupní a výstupní pupila]] [[de:Eintrittspupille]]