Entrance pupil
From Wikipedia, the free encyclopedia
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 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 aperture itself).
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 (f/8.3) in a very brightly lit place to 8 mm (f/2.1) in the dark.[1]
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.[2] [3] As a result, the centre of the pupil is sometimes called the "no-parallax point" of the lens.[4]
[edit] See also
[edit] References
- ^ Hecht, Eugene (1987). Optics, 2nd ed., Addison Wesley. ISBN 0-201-11609-X.
- ^ Kerr, Douglas A. (2005). The Proper Pivot Point for Panoramic Photography (PDF). The Pumpkin. Retrieved on 2007-01-14.
- ^ van Walree, Paul. Misconceptions in photographic optics. Retrieved on 2007-01-14. Item #6.
- ^ Littlefield, Rik (2006-02-06). "Theory of the “No-Parallax” Point in Panorama Photography" (pdf). ver. 1.0. Retrieved on 2007-01-14.
[edit] External links
- The Grid — Alain Hamblenne's method for a precise location of the entrance pupil on a DSLR camera
