In photography, aperture is the mechanism inside a lens which allows photographers to adjust the diameter of the aperture to determine the amount of light reaching the film plane or image sensor. The aperture not only affects picture brightness but also controls depth of focus. In order to control the opening and closing of the aperture, a special element known as the diaphragm is placed in the optical path. The size of the opening diaphragm is called f-stop or aperture value or just stop.

An f-stop is the ratio of focal length to to effective aperture diameter. The photographic term “one f-stop” refers to a factor of √2, approximately 1.41, which is equivalent to a factor of 2 changes in light intensity. In practice, the aperture of a lens is usually expressed as a range of f-stops. Typical ranges of f-stops use in photography usually cover 6 stops, which may be divided into wide, middle, and narrow of 2 stops each, roughly (using round numbers) f/2–f/4, f/4–f/8, and f/8–f/16 or (for a slower lens)f/2.8–f/5.6, f/5.6–f/11, and f/11–f/22. These are not sharp divisions, and ranges for specific lenses vary.

As a matter of performance, lenses often do not perform optimally when fully opened. They generally have better sharpness when stopped down couple f-stops – note that this is sharpness in the plane of critical focus, setting aside issues of depth of field. Beyond a certain point there is no further sharpness benefit to stopping down, and the diffraction begins to become significant. There is accordingly a sweet spot, generally in the f/4 –f/8 range, depending on camera, where sharpness is optimal, though some lenses are designed to perform optimally when wide open. How significant this is varies between lenses, and opinions differ on how much practical impact this has.

According to Rudolph Kingslake, who wrote “A History of the Photographic Lens,” London: Academic Press, 1989, the inventor of the iris diaphragm is unknown. Others credit Joseph Nicéphore Niépce for this device, around 1820. J. H. Brown, a member of the Royal Microscopical Society, appears to have invented a popular improved iris diaphragm by 1867.

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• References

• Wikipedia contributors, “Aperture,” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Aperture&oldid=440675603 (accessed July 21, 2011).

• Wikipedia contributors, “Diaphragm (optics),” Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Diaphragm_(optics)&oldid=436449319 (accessed July 21, 2011).