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Entrance pupil

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For the part of the eye, see Pupil.
A camera lens adjusted for large and small 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.
File:030608 Pupil.jpg
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 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) of the aperture stop in the optics that come before it. In a camera, the aperture stop is the 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 actual aperture itself). That is, EN = f/N, where EN is the diameter of the entrance pupil, f is the focal length, and N is the f-number ("relative aperture").[1]

The entrance pupil of the eye, which is not quite the same as the physical pupil, is typically about 4 mm in diameter. It can range from 2 mm (Template:F/) in a very brightly lit place to 8 mm (Template:F/) in the dark.[2]

Depending on the lens design, the entrance pupil's location on the optical axis may be behind, within or in front of the lens system, or even at infinity in the case of telecentric systems. 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 stitched together into a panorama.[3][4] As a result, the centre of the pupil is sometimes called the "no-parallax point" of the lens.[5]

See alsoEdit


  1. Jacobson, Ralph (et al.) (1988). The Manual of Photography, 8th ed., Focal Press. ISBN 0-240-51268-5. p.49
  2. Hecht, Eugene (1987). Optics, 2nd ed., Addison Wesley. ISBN 0-201-11609-X.
  3. Kerr, Douglas A. (2005). The Proper Pivot Point for Panoramic Photography. (PDF) The Pumpkin. URL accessed on 2007-01-14.
  4. van Walree, Paul. Misconceptions in photographic optics. URL accessed on 2007-01-14. Item #6.
  5. Littlefield, Rik (2006-02-06). "Theory of the “No-Parallax” Point in Panorama Photography" (pdf). ver. 1.0. Retrieved on 2007-01-14.

External linksEdit

  • The Grid — Alain Hamblenne's method for a precise location of the entrance pupil on a DSLR camera
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