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Note added at 53 mins (2019-01-10 21:23:05 GMT)
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Bifocal Lenses
The traditional Bifocal lens includes both near and far distances, and includes a segment line that separates the two. The upper section of the lens includes the distance prescription, while the lower section of the lens includes a nearer, reading prescription.
https://www.coastal.com/imageresources/Landing/2015/Lenses/b...
https://www.coastal.com/eyeglasses-lenses/progressives

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Note added at 56 mins (2019-01-10 21:26:29 GMT)
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Presbyopia, Why Bifocals?
It happens to almost everyone at middle age. You begin to hold the newspaper farther away to read. Fine objects close to you look blurred. This is Presbyopia.

Bifocals are two lenses in one. One part, or segment (usually the bottom), is made to adjust the eye for close focus, and the other (top) has a different strength for distant focus. The key to wearing bifocals is to learn to automatically look through the proper part of the glass. Bifocals are somewhat difficult to get used to at first for several reasons:

Bifocals have come a long way since Benjamin Franklin invented them in the 1700s. For people concerned about their appearance, “invisible” bifocals or bifocal contact lenses may be an option.

https://chicago.medicine.uic.edu/departments/academic-depart...