Apochromat


Apochromat

An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better color correction than the much more common "achromat" lenses. 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. Astronomers face similar problems, particularly with telescopes that use lenses rather than mirrors. "Achromatic" lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. "Apo"chromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. [ [http://stason.org/TULARC/recreation/photography/lenses-faq/31-What-do-APO-and-Apochromatic-mean.html What do APO and Apochromatic mean?] ] 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.Astronomical objectives for wide-band digital imaging 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 mm aperture range have been developed and marketed by several different firms, with focal ratios 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 APO 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 APO lenses have been produced for medium format, digital and 35 mm cameras.

Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. This is usually achieved using costly fluoro-crown glasses, abnormal flint glasses, 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.

ee also

* Achromat
* Superachromat

References

External links

* [http://www.dpreview.com/learn/?/Glossary/Optical/chromatic_aberration_01.htm Dpreview's Chromatic Aberration related page]


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Look at other dictionaries:

  • Apochromat — Als Apochromat (griech. für frei von Farben, farblos) bezeichnet man ein optisches System, z. B. ein Objektiv, bei dem der Farblängsfehler weitestgehend korrigiert ist, also eine sehr geringe Variation der Schnittweite mit der Wellenlänge… …   Deutsch Wikipedia

  • Apochromat — Apochromat, photographisches Objektiv, bei dem der Farbenfehler derart[248] behoben ist, daß nicht nur zwei, sondern mindestens drei farbige Strahlengattungen (Rotorange, Grün, Blauviolett) in denselben Brennpunkt fallen (s. Achromate).… …   Lexikon der gesamten Technik

  • Apochromat — Apochromāt (grch.) ein Linsensystem, welches frei von sekundärer Farbenzerstreuung und dabei zugleich für mehrere Farben aplanatisch ist …   Kleines Konversations-Lexikon

  • Apochromat — apochromatas statusas T sritis fizika atitikmenys: angl. apochromat vok. Apochromat, m rus. апохромат, m pranc. apochromat, m …   Fizikos terminų žodynas

  • apochromat — apochromatas statusas T sritis fizika atitikmenys: angl. apochromat vok. Apochromat, m rus. апохромат, m pranc. apochromat, m …   Fizikos terminų žodynas

  • Apochromat — apochromatinis objektyvas statusas T sritis fizika atitikmenys: angl. apochromat; apochromatic lens; apochromatic objective vok. Apochromat, m; apochromatisches Objektiv, n rus. апохромат, m; апохроматический объектив, m; объектив апохромат, m… …   Fizikos terminų žodynas

  • apochromat — apochromatinis objektyvas statusas T sritis fizika atitikmenys: angl. apochromat; apochromatic lens; apochromatic objective vok. Apochromat, m; apochromatisches Objektiv, n rus. апохромат, m; апохроматический объектив, m; объектив апохромат, m… …   Fizikos terminų žodynas

  • Apochromat — Apo|chro|mat 〈[ kro ] m. 1; Opt.〉 Linsensystem, bei dem die chromatische Aberration für mindestens drei Farben u. die sphärische Aberration für zwei Farben korrigiert wurde, bes. verwendet bei Fernsehobjektiven, Mikroskopen u. in der… …   Universal-Lexikon

  • Apochromat — Objektiv, bei dem die chromatische Aberration (Farbfehler) für alle drei Grundfarben korrigiert ist. Erreicht wird dies durch eine Linse einer speziellen Glassorte, die etwa mit einem Zusatz von Metallsalzen (z.B. Lanthanschwefelfluorit)… …   Das Lexikon aus „Bernie's Foto-Programm"

  • Apochromat — Апохромат (объектив, исправленный на хроматическую аберрацию) …   Краткий толковый словарь по полиграфии


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