In an optical system, the entrance pupil is the optical image of the physical

aperture stop In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image of ...

, as 'seen' through the optical elements in front of the stop. The corresponding image of the aperture stop as seen through the optical elements behind it is called the ''

exit pupil 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 optic ...

''. The entrance pupil defines the cone of rays that can enter and pass through the optical system. Rays that fall outside of the entrance pupil will not pass through the system. If there is no lens in front of the aperture (as in a

pinhole camera A pinhole camera is a simple camera without a lens but with a tiny aperture (the so-called ''Pinhole (optics), pinhole'')—effectively a light-proof box with a small hole in one side. Light from a scene passes through the aperture and projects a ...

), 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 or diminished image of the aperture that is displaced from the aperture location. The entrance pupil is usually a

virtual image In optics, the ''image'' of an object is defined as the collection of Focus (optics), focus points of Ray (optics), light rays coming from the object. A ''real image'' is the collection of focus points made by Vergence (optics), converging ray ...

: it lies behind the first optical surface of the system. The entrance pupil is a useful concept for determining the size of the cone of rays that an optical system will accept. Once the size and the location of the entrance pupil of an optical system is determined, the maximum cone of rays that the system will accept from a given object plane is determined solely by the size of the entrance pupil and its distance from the object plane, without any need to consider ray refraction by the optics. In photography, the size of the entrance pupil (rather than the size of the physical aperture stop) is used to calibrate the opening and closing of the diaphragm aperture. The

f-number An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical ...

(also called the ), , is defined by , where is the

focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...

and 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 center of the entrance pupil is the vertex of a camera's

angle of view In photography, angle of view (AOV) describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term '' field of view''. It is important to distinguish the angle of view from the ...

as chief rays cross this point. Consequently, this point is the camera's center of perspective, perspective point, viewpoint, projection center or no-parallax point. This point is important in

panoramic photography Panoramic photography is a technique of photography, using specialized equipment or software, that captures images with horizontally elongated field of view, fields of view. It is sometimes known as ''wide format photography''. The term has also ...

without digital image processing, because the camera must be rotated around the center of the entrance pupil to avoid

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

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. The entrance pupil of the human eye, which is not quite the same as the physical pupil, is typically about in diameter. It can range from () in a very brightly lit place to () in the dark.{{cite book , first=Eugene, last=Hecht, year=1987, title=Optics, edition=2nd, publisher=Addison Wesley, isbn=0-201-11609-X An optical system is typically designed with a single aperture stop, and therefore has a single entrance pupil at designed working conditions. In general, though, the determination of which element is the aperture stop depends on the object distance, so a system may have different entrance pupils for different object planes. Similarly, vignetting can cause different lateral locations at a given object plane to have different aperture stops, and therefore different entrance pupils.

References

External links

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

RP Photonics Encyclopedia Optics Science of photography

See also

References

External links