Entrance pupil Totally Explained

Everything about Entrance Pupil totally explained

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 aperture itself).
   The entrance pupil of the eye, which isn't quite the same as the physical pupil, 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.
   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 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. As a result, the centre of the pupil is sometimes called the "no-parallax point" of the lens.

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