a lens with two principal focal lengths: the upper part of the lens gives a sharp image of distant objects and the lower part gives a sharp image of near objects, as when reading. Examples are bifocal spectacles, bifocal contact lenses, and bifocal intraocular lenses. See also trifocal lenses, multifocal lenses.
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a lens that has two segments with different refractive powers, ordinarily with the upper for distant and the lower for near vision; used in bifocal glasses.Medical dictionary. 2011.