Apochromatic is a description of a photographic or other lens having a high degree of color correction. Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens (see the article for an excellent illustration). In photography, it leads to fuzzy images, and to color fringes at high-contrast edges, like an edge between black and white. Astronomers face similar problems, particularly with telescopes that use lenses rather than mirrors.
Opticalengineers can design lenses that bring several colors to a focus at the same distance from the lens. Such lenses can be apochromatic, and can give sharper images than uncorrected optics. However, it requires extra pieces of glass in the optical path, and additional kinds of glass. This makes apochromatic lenses heavier and more expensive than their uncorrected counterparts.
An apochromatic lens is often called an apochromat, or apo, for short.
Dpreview's Chromatic Aberration related page (http://www.dpreview.com/learn/?/Glossary/Optical/Chromatic_Aberrations_01.htm)
This invention relates generally to apochromatic lens triplets of the type in which a fluidal liquid lens element is contained between two identical rigid lens elements, and more particularly to a technique for optimizing performance of such a lens triplet by providing aspherical lens surfaces.
Apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane.
Apochromats are also corrected for spherical aberration at two wavelengths, rather than one as in an achromat.
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 to f/7.
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