aperture

[ap-er-cher]
noun
1.
an opening, as a hole, slit, crack, gap, etc.
2.
Also called aperture stop. Optics. an opening, usually circular, that limits the quantity of light that can enter an optical instrument.

Origin:
1400–50; late Middle English < Latin apertūra an opening, equivalent to apert(us) opened (past participle of aperīre; aper(i)- (see aperient) + -tus past participle suffix) + -ūra -ure

apertural [ap-er-choor-uhl] , adjective
apertured, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
aperture (ˈæpətʃə)
 
n
1.  a hole, gap, crack, slit, or other opening
2.  physics
 a.  a usually circular and often variable opening in an optical instrument or device that controls the quantity of radiation entering or leaving it
 b.  See also relative aperture the diameter of such an opening
 
[C15: from Late Latin apertūra opening, from Latin aperīre to open]

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Etymonline
Word Origin & History

aperture
1640s, from L. apertura "an opening," from apertus, pp. of aperire "to open" (see overt).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

aperture ap·er·ture (āp'ər-chər)
n.

  1. An opening, such as a hole, gap, or slit.

  2. A usually adjustable opening in an optical instrument, such as a microscope, a camera, or a telescope, that limits the amount of light passing through a lens or onto a mirror.

  3. The diameter of such an opening.

  4. The diameter of the objective of a telescope or microscope.


ap'er·tur'al adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Encyclopedia Britannica
Encyclopedia

aperture

in optics, the maximum diameter of a light beam that can pass through an optical system. The size of an aperture is limited by the size of the mount holding the optical component, or the size of the diaphragm placed in the bundle of light rays. The hole in the mount or diaphragm that limits the size of the aperture is called an aperture stop. Thus, an aperture stop determines the amount of light that traverses an optical system and hence determines the image illumination

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Another factor that affects what shutter speed and aperture combination is best
  is the film that is used.
Every picture is captioned with the exposure time, aperture setting and film
  brand and speed.
If your camera doesn't have portrait mode, use aperture-priority mode.
The iris is a circular diaphragm behind the cornea, and presents near its
  center a rounded aperture, the pupil.
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