Exit pupil

Exit pupil

In optics, the exit pupil is a virtual aperture in an optical system. Only 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 telescope or compound microscope, this image is the image of the objective element(s) as produced by the eyepiece. The size and shape of this disc is crucial to the instrument's performance, because the observer's eye can see light only if it passes through this tiny aperture. The term "exit pupil" is also sometimes used to refer to the diameter of the virtual aperture. Older literature on optics sometimes refers to the exit pupil as the "Ramsden disc", named after English instrument-maker Jesse Ramsden.

Visual instruments

To use an optical instrument, the entrance pupil of the viewer's eye must be aligned with and be of similar size to the instrument's exit pupil. This properly couples the optical system to the eye and avoids vignetting. (The entrance pupil of the eye is the image of the anatomical pupil as seen through the cornea.) The location of the exit pupil thus determines the eye relief of an eyepiece. Good eyepiece designs produce an exit pupil of diameter approximating the eye's apparent pupil diameter, and located about 20 mm away from the last surface of the eyepiece for the viewer's comfort. If the disc is much larger than the eye's pupil, much of the light will be lost instead of entering the eye; if smaller, the view will be vignetted. If the disc is too close to the last surface of the eyepiece, the eye will have to be uncomfortably close for viewing; if too far away, the observer will have difficulty maintaining the eye's alignment with the disc.

Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application. An astronomical telescope requires a large pupil because it is designed to be used for looking at dim objects at night, while a microscope will require a much smaller pupil since the object will be brightly illuminated. A set of 7×50 binoculars has an exit pupil just over 7 mm, the average human eye pupil size at night. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission). In daylight, when the pupil is only 4 mm in diameter, over half the light will not reach the retina. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That is just enough to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use.

The optimum eye relief distance also varies with application. For example a rifle scope needs a very long eye relief to prevent recoil from causing it to strike the observer.

The exit pupil can be visualized by focusing the instrument on a bright, nondescript field, and holding a white card up to the eyepiece. This projects a disc of light onto the card. By moving the card closer to or further away from the eyepiece, the disc of light will be minimized when the card is at the exit pupil, and the bright disc then shows the diameter of the pupil. A clear vial of milky fluid can also be used to visualize the light rays, which appear as an hourglass shape converging and diverging as they exit the eyepiece, with the smallest cross-section (the waist of the hourglass shape) representing the exit pupil.


For a telescope, the diameter of the exit pupil can be calculated by dividing the focal length of the eyepiece by the focal ratio (f-number) of the telescope. More simply, it is the diameter of the objective lens divided by the magnification.

Digital photography

The concept of exit pupil is not often discussed in books on photographic optics, but has become a very important topic for digital cameras. The distance of the exit pupil from the sensor plane determines the range of angles of incidence that light will make with the sensor. Image sensors often have a limited range of angles over which they will efficiently accept light, especially those that use microlenses to increase their sensitivity.cite web |url=http://www.swissarmyfork.com/digital_lens_faq.htm |title=The Digital Lens FAQ |accessdate=2008-05-11 |last=Wisniewski |first=Joseph S. |date=2003-12-06 ] The closer the exit pupil to the focal plane, the higher the angles of incidence at the extreme edges of the field. This can lead to pixel vignetting.

ee also

* Transmittance
* Diaphragm (optics)
* Pupil magnification


*cite book | first=John E. | last=Greivenkamp | year=2004 | title=Field Guide to Geometrical Optics | publisher=SPIE | others=SPIE Field Guides vol. FG01 | id=ISBN 0-8194-5294-7
*cite book | first=Eugene|last=Hecht|year=1987|title=Optics|edition=2nd ed.|publisher=Addison Wesley|id=ISBN 0-201-11609-X

External links

* [http://www.astronomics.com/main/definition.asp/catalog_name/Astronomics/category_name/71R042EX658J8LNK80AKKWHUF5/Page/1 A short definition of relative brightness]

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