In an

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. The corresponding image of the aperture as seen through the back of the lens system is called the exit pupil. If there is no lens in front of the aperture (as in a pinhole camera), the entrance pupil's location and size are identical to those of the aperture. Optical elements in front of the aperture will produce a

magnified ''Magnified'' is the second album by the American alternative rock band Failure. Production Drummer Robert Gauss departed during the recording of the album; the remaining drum parts were played by John Dargahi and Greg Edwards. ''Magnified'' dif ...

or diminished image that is displaced from the location of the physical aperture. The entrance pupil is usually a virtual image: it lies behind the first optical surface of the system. The geometric location of the entrance pupil is the vertex of the camera's

angle of view The angle of view is the decisive variable for the visual perception of the size or projection of the size of an object. Angle of view and perception of size The perceived size of an object depends on the size of the image projected onto the ...

and consequently its center of perspective, perspective point, view point, projection centre or no-parallax point. This point is important in panoramic photography, because the camera must be rotated around it in order to avoid parallax errors in the final, stitched panorama. Panoramic photographers often incorrectly refer to the entrance pupil as a nodal point, which is a different concept. Depending on the lens design, the entrance pupil location on the optical axis may be behind, within or in front of the lens system; and even at infinite distance from the lens in the case of telecentric systems. In photography, the size of the entrance pupil (rather than the size of the physical aperture itself) is used to calibrate the opening and closing of the diaphragm aperture. The

f-number In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill ...

("relative aperture"), ''N'', is defined by ''N = f/E_N'', where ''f'' is the focal length and ''E''_N is the diameter of the entrance pupil. Increasing the focal length of a lens (i.e., zooming in) will usually cause the f-number to increase, and the entrance pupil location to move further back along the optical axis. The entrance pupil of the human

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

, which is not quite the same as the physical

pupil The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...

, is typically about 4 mm in diameter. It can range from 2 mm () in a very brightly lit place to 8 mm () in the dark.{{cite book , first=Eugene, last=Hecht, year=1987, title=Optics, edition=2nd, publisher=Addison Wesley, isbn=0-201-11609-X

References

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Stops and Pupils
in ''Field Guide to Geometrical Optics'' Greivenkamp, John E, 2004 Optics Science of photography

See also

References

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