Progressive lenses, also called progressive addition lenses or progressive power 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 disruption to the visual field that occurs in bifocal and trifocal lenses. They suffer the disadvantage of creating regions of distortion and blur off the optic axis of the lens.
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