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==Construction== {{Main|Photographic lens design|History of photographic lens design}} [[Image:Telephoto.jpg|thumb|The [[zoom lens]] assembly of the [[Canon ELPH (camera)|Canon Elph]]]] A camera lens may be made from a number of elements: from one, as in the [[Brownie (camera)|Box Brownie]]'s meniscus lens, to over 20 in the more complex zooms. These elements may themselves comprise a group of lenses cemented together. The front element is critical to the performance of the whole assembly. In all modern lenses the surface is coated to reduce abrasion, [[Lens flare|flare]], and [[Anti-reflective coating|surface reflectance]], and to adjust color balance. To minimize aberration, the curvature is usually set so that the [[angle of incidence (optics)|angle of incidence]] and the [[angle of refraction]] are equal. In a prime lens this is easy, but in a zoom there is always a compromise. The lens usually is [[focus (optics)|focused]] by adjusting the distance from the lens assembly to the image plane, or by moving elements of the lens assembly. To improve performance, some lenses have a cam system that adjusts the distance between the groups as the lens is focused. Manufacturers call this different things: [[Nikon]] calls it CRC (close range correction); [[Canon Inc.|Canon]] calls it a floating system; and [[Hasselblad]] and [[Mamiya]] call it FLE (floating lens element).<ref name="photonotes">{{cite web|url=http://photonotes.org/cgi-bin/entry.pl?id=Floatingelement|title=PhotoNotes.org Dictionary β Floating element|publisher=photonotes.org|access-date=2014-10-25|archive-url=https://web.archive.org/web/20140810223927/http://photonotes.org/cgi-bin/entry.pl?id=Floatingelement#|archive-date=2014-08-10|url-status=dead}}</ref> Glass is the most common material used to construct lens elements, due to its good optical properties and resistance to scratching. Other materials are also used, such as [[quartz glass]], [[fluorite]],<ref>{{Cite web | url=http://www.ukaoptics.com/uvquartz.html | title=Ultraviolet Quartz Lenses | publisher=Universe Kogaku | access-date=2007-11-05 | archive-date=2007-11-25 | archive-url=https://web.archive.org/web/20071125155248/http://www.ukaoptics.com/uvquartz.html | url-status=live }}</ref><ref>{{Cite web | url=http://www.canon.com/camera-museum/tech/room/hotaru.html | title=Technical Room β Fluorite / UD / Super UD glass Lenses | publisher=[[Canon (company)|Canon]] | access-date=2007-11-05 | archive-url=https://web.archive.org/web/20090530072058/http://www.canon.com/camera-museum/tech/room/hotaru.html# | archive-date=2009-05-30 | url-status=dead }}</ref><ref>{{Cite web |url=http://cpn.canon-europe.com/content/education/infobank/lenses/fluorite_aspherical_and_ud_lenses.do |title=Lenses: Fluorite, aspherical and UD lenses |work=Canon Professional Network |access-date=2008-10-04 |archive-date=2011-08-14 |archive-url=https://web.archive.org/web/20110814012805/http://cpn.canon-europe.com/content/education/infobank/lenses/fluorite_aspherical_and_ud_lenses.do |url-status=live }}</ref><ref>{{Cite web | url=http://www.macrolenses.de/objektive_sl.php?lang | title=The Macrolens Collection Database | first=Klaus | last=Gottermeier | access-date=2007-11-05 | archive-date=2008-01-17 | archive-url=https://web.archive.org/web/20080117114550/http://www.macrolenses.de/objektive_sl.php?lang | url-status=live }}</ref> plastics like [[Acrylic glass|acrylic]] (Plexiglass), and even [[germanium]] and [[meteorite|meteoritic glass]].{{citation needed|date=January 2011}} Plastics allow the manufacturing of strongly [[aspheric lens|aspherical lens elements]] which are difficult or impossible to manufacture in glass, and which simplify or improve lens manufacturing and performance.{{citation needed|date=January 2011}} Plastics are not used for the outermost elements of all but the cheapest lenses as they scratch easily. Molded plastic lenses have been used for the cheapest disposable cameras for many years, and have acquired a bad reputation: manufacturers of quality optics tend to use euphemisms such as "optical resin". However many modern, high performance (and high priced) lenses from popular manufacturers include molded or hybrid aspherical elements, so it is not true that all lenses with plastic elements are of low photographic quality.{{Citation needed|date=November 2019|reason=These lenses contain molded GLASS elements, not plastic ones; I do not know of any high-quality lens which uses a plastic or "optical resin" element.}} The [[1951 USAF resolution test chart]] is one way to measure the resolving power of a lens. The quality of the material, coatings, and build affect the resolution. Lens resolution is ultimately limited by [[diffraction]], and very few photographic lenses approach this resolution. Ones that do are called "diffraction limited" and are usually extremely expensive.<ref name="luminous-landscape">{{cite web|url=http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml|title=Understanding Lens Diffraction|publisher=luminous-landscape.com|access-date=2014-10-25|archive-url=https://web.archive.org/web/20141025223233/http://www.luminous-landscape.com/tutorials/understanding-series/u-diffraction.shtml|archive-date=2014-10-25|url-status=dead}}</ref> Today, most lenses are [[Optical coating|multi-coated]] in order to minimize [[lens flare]] and other unwanted effects. Some lenses have a UV coating to keep out the [[ultraviolet]] light that could taint color. Most modern optical cements for bonding glass elements also block UV light, negating the need for a UV filter. However, this leaves an avenue for [[lens fungus]] to attack if lenses are not cared for appropriately. UV photographers must go to great lengths to find lenses with no cement or coatings. A lens will most often have an aperture adjustment mechanism, usually an [[Diaphragm (optics)|iris diaphragm]], to regulate the amount of light that passes. In early camera models a rotating plate or slider with different sized holes was used. These [[Waterhouse stop]]s may still be found on modern, specialized lenses. A [[shutter (photography)|shutter]], to regulate the time during which light may pass, may be incorporated within the lens assembly (for better quality imagery), within the camera, or even, rarely, in front of the lens. Some cameras with leaf shutters in the lens omit the aperture, and the shutter does double duty.
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