View source for Apochromat

{{Short description|Type of photographic or other lens}}
{{Refimprove|date=October 2009}}
[[Image:Chromatic_aberration_lens_diagram.svg|thumb|270px|right|Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths.]]
An '''apochromat''', or '''apochromatic lens''' ('''apo'''), is a [[Photographic lens|photographic]] or other [[Lens (optics)|lens]] that has better correction of [[chromatic aberration|chromatic]] and [[spherical aberration]] than the much more common [[achromat]] lenses.

The prefix ''apo-'' comes from the Greek preposition ''ἀπό-'', meaning free from or away from.

==Explanation==
[[Chromatic aberration]] is the phenomenon of different colors focusing at different distances from a lens. In photography, chromatic aberration produces soft overall images, and color fringing at high-contrast edges, like an edge between black and white. [[Astronomer]]s face similar problems, particularly with [[telescopes]] that use lenses rather than [[mirror]]s. ''Achromatic'' lenses are corrected to bring ''two'' [[wavelength]]s into [[focus (optics)|focus]] in the same plane – typically red (~0.590&nbsp;[[micrometre|μm]]) and blue (~0.495&nbsp;[[micrometre|μm]]). ''Apo''chromatic lenses are designed to bring ''three'' colors into focus in the same plane – typically red (~0.620&nbsp;[[micrometre|μm]]), green (~0.530&nbsp;[[micrometre|μm]]), and blue (~0.465&nbsp;[[micrometre|μm]]).<ref>{{cite web |url=http://stason.org/TULARC/recreation/photography/lenses-faq/31-What-do-APO-and-Apochromatic-mean.html |title=What do APO and Apochromatic mean? |website=stason.org |series=Lenses FAQ}}</ref> The residual color error (secondary spectrum) can be up to an order of magnitude less than for an achromatic lens of equivalent aperture and focal length. Apochromats are also corrected for [[spherical aberration]] at two wavelengths, rather than one as in an achromat.
[[Image:Apochromatic focal curve.svg|thumb|275px|left|Apochromatic lens brings three colors to a common focal plane. Notice that this lens is designed for astronomy, not viewing, since one of the wavelengths (~0.780&nbsp;[[micrometre|μm]]) is in the [[near infrared]], outside the visible spectrum.]]
Telescope objective lenses for wide-band digital imaging in astronomy must have apochromatic correction, as the optical sensitivity of typical CCD imaging arrays can extend from the [[ultraviolet]] through the [[visible spectrum]] and into the [[near infrared]] wavelength range. Apochromatic lenses for astrophotography in the 60–150&nbsp;mm aperture range have been developed and marketed by several firms, with [[f-number|focal ratio]]s ranging from {{f/|5|link=yes}} to {{f/}}7. Focused and guided properly during the exposure, these apochromatic objectives are capable of producing the sharpest wide-field astrophotographs optically possible for the given aperture sizes.

[[Graphic arts]] process (copy) cameras generally use apochromatic lenses for sharpest possible imagery as well. Classically designed apochromatic process camera lenses generally have a maximum aperture limited to about {{f/}}9. More recently, higher-speed apochromatic lenses have been produced for medium format, digital and 35&nbsp;mm cameras.

[[File:Apochromat.svg|alt=Apochromat lens.svg|thumb|right|The Apochromatic lens usually comprises three elements that bring light of three distinct colors to a common focus]]
Apochromatic designs require optical glasses with special [[dispersion (optics)|dispersive]] properties to achieve three color crossings. This is usually achieved using costly fluoro-[[Crown glass (optics)|crown glass]]es, abnormal [[flint glass]]es, and even optically transparent liquids with highly unusual dispersive properties in the thin spaces between glass elements. The temperature dependence of glass and liquid [[index of refraction]] and dispersion must be accounted for during apochromat design to assure good optical performance over reasonable temperature ranges with only slight re-focusing. In some cases, apochromatic designs without anomalous dispersion glasses are possible.

==Use in photography==
Independent tests can be used to demonstrate that the "APO" designation is used rather loosely by some photographic lens manufacturers to describe the [[Chromatic aberration|color accuracy]] of their lenses, as comparable lenses have shown superior color accuracy even though they did not carry the "APO" designation.<ref>{{Cite web |url=http://www.slrgear.com/reviews/showproduct.php/product/227/cat/30 |title=A 300mm f/2.8 photographic lens with the "APO" designation, tested for chromatic aberration |access-date=2011-03-28 |archive-date=2015-09-10 |archive-url=https://web.archive.org/web/20150910180225/http://www.slrgear.com/reviews/showproduct.php/product/227/cat/30 |url-status=dead }}</ref><ref>{{Cite web |url=http://www.slrgear.com/reviews/showproduct.php/product/164/cat/10 |title=A 300mm f/2.8 photographic lens without the "APO" designation, tested for chromatic aberration |access-date=2011-03-28 |archive-date=2015-09-11 |archive-url=https://web.archive.org/web/20150911232238/http://www.slrgear.com/reviews/showproduct.php/product/164/cat/10 |url-status=dead }}</ref>

Also, when considering lens design, the "APO" designation is used more conservatively in astronomy-related optics (e.g. telescopes) and microscopy than in photography. For example, telescopes that are marked "APO" are specialized, [[Prime lens|fixed focal length lenses]] that are optimised for infinity-like distances whereas in photography, even certain relatively low-priced general-purpose [[zoom lens]]es are given the APO designation.<ref>{{Cite web |url=http://www.sigmaphoto.com/shop/70-300mm-f4-56-apo-dg-macro-sigma |title=An inexpensive photographic zoom lens with the "APO" designation |access-date=2011-03-25 |archive-url=https://web.archive.org/web/20110310010716/http://www.sigmaphoto.com/shop/70-300mm-f4-56-apo-dg-macro-sigma |archive-date=2011-03-10 |url-status=dead }}</ref>

Often, however, apochromatic lenses used in fine cameras are not termed apochromats, Instead, they may be simply called "fluorite lenses", based on the material with anomalous partial dispersion which allowed them to be apochromatic. Such lenses began to be available to photographers in 1969, with the Canon FL-F 300mm f/5.6 telephoto lens. Fluorite has some drawbacks, for example vulnerability to sudden changes in temperature, and thus attempts were made to use substitutes, such as fluorophosphate glasses, which ameliorate, but do not completely eliminate (as compared with ordinary glass) these drawbacks.

==See also==
{{Comparison_chromatic_focus_shift_plots.svg}}
* [[Superachromat]]
* [[Fluorite lens]]
* [[List of telescope types]]

==References==
{{reflist}}

[[Category:Lenses]]
[[Category:Microscope components]]