Editing Camera lens (section)

==Aperture and focal length==
[[File:Lenses with different apertures.jpg|thumb|upright|[[Apertures]] in 1-stop increments on the same lens.]]
[[File:Focal length.jpg|thumb|right|How focal length affects photograph composition: adjusting the camera's distance from the main subject while changing focal length, the main subject can remain the same size, while the other at a different distance changes size.]]
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The two fundamental parameters of an optical lens are the [[focal length]] and the maximum [[aperture]]. The lens' focal length determines the magnification of the image projected onto the image plane, and the aperture the light intensity of that image. For a given photographic system the focal length determines the [[angle of view]], short focal lengths giving a wider field of view than longer focal length lenses. A wider aperture, identified by a smaller f-number, allows using a faster shutter speed for the same exposure.  The [[F-number#Camera_equation_(G#)|camera equation]], or G#, is the ratio of the [[radiance]] reaching the camera sensor to the [[irradiance]] on the focal plane of the camera lens.<ref name="g-number">{{cite book |last1=Driggers |first1=Ronald G. |title=Encyclopedia of Optical Engineering: Pho-Z, pages 2049-3050 |date=2003 |publisher=CRC Press |isbn=978-0-8247-4252-2 |url=https://books.google.com/books?id=rcrGlrguj1YC |access-date=18 June 2020 |language=en |archive-date=27 October 2023 |archive-url=https://web.archive.org/web/20231027111815/https://books.google.com/books?id=rcrGlrguj1YC |url-status=live }}</ref>

The maximum usable aperture of a lens is specified as the focal ratio or [[f-number]], defined as the lens's [[focal length]] divided by the effective aperture (or [[entrance pupil]]), a dimensionless number. The lower the f-number, the higher light intensity at the focal plane. Larger apertures (smaller f-numbers) provide a much shallower [[depth of field]] than smaller apertures, other conditions being equal. Practical lens assemblies may also contain mechanisms to deal with measuring light, secondary apertures for flare reduction,<ref name="mir">{{cite web|url=http://www.mir.com.my/rb/photography/hardwares/classics/eos/EF-lenses/EF2035mmf3545USM/index.htm|title=Canon EF 20-35mm f/3.5~4.5 USM – Index Page|publisher=mir.com.my|access-date=2014-10-25|archive-date=2014-10-25|archive-url=https://web.archive.org/web/20141025192929/http://mir.com.my/rb/photography/hardwares/classics/eos/EF-lenses/EF2035mmf3545USM/index.htm|url-status=live}}</ref> and mechanisms to hold the aperture open until the instant of exposure to allow [[single-lens reflex camera|SLR]] cameras to focus with a brighter image with shallower depth of field, theoretically allowing better focus accuracy.

Focal lengths are usually specified in millimetres (mm), but older lenses might be marked in centimetres (cm) or inches. For a given film or sensor size, specified by the length of the diagonal, a lens may be classified as a:

* [[Normal lens]]: angle of view of the diagonal about 50° and a focal length approximately equal to the image diagonal. <!-- It's generally accepted that 50mm is a bit longer than "normal" for 35mm film; I've based this on 75mm with 6×4cm. Maybe the angle of width, rather than diagonal, is better? -->
* [[Wide-angle lens]]: angle of view wider than 60° and focal length shorter than normal.
* [[Long-focus lens]]: any lens with a focal length longer than the diagonal measure of the film or sensor.<ref name="google2">{{cite book|title=Applied Photographic Optics: Lenses and Optical Systems for Photography, Film, Video, Electronic and Digital Imaging|author=Ray, S.F.|date=2002|publisher=Focal|isbn=9780240515403|url=https://books.google.com/books?id=cuzYl4hx-B8C|page=294|access-date=2014-12-12}}</ref> Angle of view is narrower. The most common type of long-focus lens is the [[telephoto lens]], a design that uses special optical configurations to make the lens shorter than its focal length.

A side effect of using lenses of different focal lengths is the different distances from which a subject can be framed, resulting in a different [[Perspective (visual)|perspective]]. Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject. But the perspective will be different. With the wideangle, the hands will be exaggeratedly large relative to the head. As the focal length increases, the emphasis on the outstretched hand decreases. However, if pictures are taken from the same distance, and enlarged and cropped to contain the same view, the pictures will have identical perspective. A moderate long-focus (telephoto) lens is often recommended for portraiture because the perspective corresponding to the longer shooting distance is considered to look more flattering.

The widest aperture lens in history of photography is believed to be the [[Carl Zeiss Planar 50mm f/0.7]],<ref name=worldfastest>{{cite web|url=http://ogiroux.blogspot.com/2008/06/worlds-fastest-lens-zeiss-50mm-f07.html |title=Mutable Conclusions: World's fastest lens: Zeiss 50mm f/0.7. |access-date=2014-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20090309005033/http://ogiroux.blogspot.com/2008/06/worlds-fastest-lens-zeiss-50mm-f07.html |archive-date=March 9, 2009 }}</ref> which was designed and made specifically for the NASA [[Apollo program|Apollo]] lunar program to capture the far side of the Moon in 1966. Three of these lenses were purchased by filmmaker [[Stanley Kubrick]] in order to film scenes in his 1975 film ''[[Barry Lyndon]]'', using candlelight as the sole light source.<ref>Guy, 2012, p 43.</ref><ref name="zeiss">{{cite web|url=http://www.zeiss.com/C12567A8003B58B9/allBySubject/B5283AEBBB27ADA3C1256A2B0037E175|title=Hollywood, NASA, and the chip industry put their trust in Carl Zeiss|publisher=zeiss.com|access-date=2014-12-12|archive-date=2010-10-01|archive-url=https://web.archive.org/web/20101001210723/http://www.zeiss.com/C12567A8003B58B9/allBySubject/B5283AEBBB27ADA3C1256A2B0037E175|url-status=live}}</ref><ref>{{Cite web |url=http://www.contaxinfo.com/pdf_files/When_is_it_advisable_Zeiss_1979.pdf# |title=Dr. J. Kämmerer «When is it advisable to improve the quality of camera lenses?» Excerpt from a lecture given during the Optics & Photography Symposium, Les Baux, 1979 |access-date=2012-10-27 |archive-url=https://web.archive.org/web/20030624111149/http://www.contaxinfo.com/pdf_files/When_is_it_advisable_Zeiss_1979.pdf# |archive-date=2003-06-24 |url-status=dead }}</ref>

{{Gallery
|align=center
|title=An example of how lens choice affects angle of view. The photos were taken by a [[135 film|35 mm]] camera at a constant distance from the subject.
|Image:Angleofview 28mm f4.jpg|28 mm lens
|Image:Angleofview 50mm f4.jpg|50 mm lens
|Image:Angleofview 70mm f4.jpg|70 mm lens
|Image:Angleofview 210mm f4.jpg|210 mm lens
}}