 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 or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses. ==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. 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. apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane.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. thumb|275px|left|apochromatic lens brings 3 wavelengths to a common focal plane. (note that this lens is designed for astronomy, so one of the 3 wavelengths is outside the visible spectrum.) 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 60â150&nbsp;mm aperture range have been developed and marketed by several different firms, with focal ratios ranging from  to 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 9. more recently, higher-speed apo lenses have been produced for medium format, digital and 35&nbsp;mm cameras. alt=apochromat lens.svg|thumb|right|the apochromatic lens is usually of three elements and brings light of three different frequencies to a common focus apochromatic designs require optical glasses with special [[d
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