Progressive lenses
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Progressive lenses, also called progressive addition lenses, progressive power lenses, and varifocal lenses, are glasses used to correct presbyopia when a person has some other refraction error such as myopia or astigmatism. A gradient of increasing lens power is added to the correction for the other refraction error, going from nothing at the top of the lens to maximum magnification at the bottom of the lens. A wearer can then adjust the lens power required for clear vision at different viewing distances by tilting his or her head to place the line of sight through different parts of the lens.
Progressive addition lenses avoid the discontinuities in the visual field created by bifocal and trifocal lenses. The lenses are also more cosmetically attractive. The lenses suffer the disadvantage of creating regions of distortion and blur away from the optic axis, yielding poor visual resolution. Although manufacturers are constantly striving to minimize these distortions, some wearers cannot tolerate the lenses.