- 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.Astronomer s face similar problems, particularly withtelescopes that use lenses rather thanmirror s. "Achromatic" lenses are corrected to bring twowavelength s (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 forspherical 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 theultraviolet through thevisible spectrum and into thenear 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 glass es, abnormalflint 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 liquidindex 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
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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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