Bifocal lenses have two parts: the upper part normally used for distance vision and the lower part used for near-vision tasks such as reading. Many people after reaching age 45 develop a condition called presbyopia, which is deterioration in the ability of the eye's natural lens to expand or contract in order to focus on close objects. People with presbyopia need a special lens for reading and may need a different lens for seeing in the distance. Benjamin Franklin invented bifocals so he wouldn't need to switch glasses when reading. They serve that same purpose today. Until recently, bifocal eyeglasses had a line going across the entire length of the lens. Now there are a number of variations on the theme. All work the same way, by reserving part of the lens for near-vision correction. The rest of the lens can be used for distance vision correction or can have no correction at all.
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Bifocal Lenses
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