Exit pupil 1118119 211800670 2008-05-12T04:15:30Z Srleffler 252195 Merge from [[Ramsden disc]]. [[Image:Camera_lens_exit_pupil.jpg|thumb|The image side of the lens of an SLR camera; the exit pupil is the light area in the middle of the lens.]] In [[optics]], the '''exit pupil''' is a virtual aperture in an optical system. Only [[ray (optics)|rays]] which pass through this virtual aperture can exit the system. The exit pupil is the [[image]] of the [[aperture stop]] in the optics that follow it. In a [[optical telescope|telescope]] or [[compound microscope]], this image is the image of the [[Objective (optics)|objective]] element(s) as produced by the [[eyepiece]]. The size and shape of this disc is crucial to the instrument's performance, because the observer's eye can see light only if it passes through this tiny aperture. The term ''exit pupil'' is also sometimes used to refer to the [[diameter]] of the virtual aperture. Older literature on optics sometimes refers to the exit pupil as the '''''Ramsden disc''''', named after English instrument-maker [[Jesse Ramsden]]. ==Visual instruments== To use an optical instrument, the [[entrance pupil]] of the viewer's [[eye]] must be aligned with and be of similar size to the instrument's exit pupil. This properly couples the optical system to the eye and avoids [[vignetting]]. (The entrance pupil of the eye is the image of the [[pupil|anatomical pupil]] as seen through the [[cornea]].) The location of the exit pupil thus determines the [[eye relief]] of an eyepiece. Good eyepiece designs produce an exit pupil of diameter approximating the eye's apparent pupil diameter, and located about 20&nbsp;mm away from the last surface of the eyepiece for the viewer's comfort. If the disc is much larger than the eye's pupil, much of the light will be lost instead of entering the eye; if smaller, the view will be vignetted. If the disc is too close to the last surface of the eyepiece, the eye will have to be uncomfortably close for viewing; if too far away, the observer will have difficulty maintaining the eye's alignment with the disc. Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application. An astronomical telescope requires a large pupil because it is designed to be used for looking at dim objects at night, while a microscope will require a much smaller pupil since the object will be brightly illuminated. A set of 7×50 [[binoculars]] has an exit pupil just over 7&nbsp;mm, the average human eye pupil size at night. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of [[brightness]] at night due to using such binoculars (assuming perfect [[Transmittance|transmission]]). In daylight, when the pupil is only 4&nbsp;mm in diameter, over half the light will not reach the [[retina]]. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That is just enough to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use. The optimum eye relief distance also varies with application. For example a [[rifle scope]] needs a very long eye relief to prevent recoil from causing it to strike the observer. The exit pupil can be visualized by focusing the instrument on a bright, nondescript field, and holding a white card up to the eyepiece. This projects a disc of light onto the card. By moving the card closer to or further away from the eyepiece, the disc of light will be minimized when the card is at the exit pupil, and the bright disc then shows the diameter of the pupil. A clear vial of milky fluid can also be used to visualize the light rays, which appear as an hourglass shape converging and diverging as they exit the eyepiece, with the smallest cross-section (the waist of the hourglass shape) representing the exit pupil. ==Telescopes== For a telescope, the diameter of the exit pupil can be calculated by dividing the [[focal length]] of the eyepiece by the [[F-number|focal ratio]] (f-number) of the telescope. More simply, it is the diameter of the [[objective lens]] divided by the [[magnification]]. ==Digital photography== The concept of exit pupil is not often discussed in books on photographic optics, but has become a very important topic for [[digital camera]]s. The distance of the exit pupil from the sensor plane determines the range of [[angle of incidence|angles of incidence]] that light will make with the sensor. [[Image sensor]]s often have a limited range of angles over which they will efficiently accept light, especially those that use [[microlens]]es to increase their sensitivity.<ref name="DLF">{{cite web |url=http://www.swissarmyfork.com/digital_lens_faq.htm |title=The Digital Lens FAQ |accessdate=2008-05-11 |last=Wisniewski |first=Joseph S. |date=[[2003-12-06]] }}</ref> The closer the exit pupil to the focal plane, the higher the angles of incidence at the extreme edges of the field. This can lead to [[pixel vignetting]]. ==See also== * [[Aperture]] * [[Vignetting]] * [[Transmittance]] * [[f-number]] * [[Diaphragm (optics)]] * [[Entrance pupil]] * [[Pupil]] * [[Pupil magnification]] ==References== <references/> *{{cite book | first=John E. | last=Greivenkamp | year=2004 | title=Field Guide to Geometrical Optics | publisher=SPIE | others=SPIE Field Guides vol. '''FG01''' | id=ISBN 0-8194-5294-7 }} *{{cite book | first=Eugene|last=Hecht|year=1987|title=Optics|edition=2nd ed.|publisher=Addison Wesley|id=ISBN 0-201-11609-X}} ==External links== *[http://www.astronomics.com/main/definition.asp/catalog_name/Astronomics/category_name/71R042EX658J8LNK80AKKWHUF5/Page/1 A short definition of relative brightness] [[Category:optics]] [[de:Austrittspupille]] [[zh:出射瞳]]