Aplanat(redirected from aplanatic)
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a photographic objective with an optical system consisting of two achromatic lenses placed symmetrically about the diaphragm. The aplanat is free from spherical aberration, chromatic aberration, and distortion; but astigmatism, though severely attenuated, remains present. Because of its simplicity of design and loose mounting requirements, the aplanat lends itself readily to use as a universal lens with a relative aperture from f/8—rarely f/5 (for portrait work and group photographs)—to f/16 (as a wide-angle lens). With the appearance of anastigmats, aplanats lost their significance and are now produced in small quantities.