Bifocals
Bifocals
Bifocals (/baɪˈfəʊkəlz/) are a type of eyeglasses that have two distinct optical powers. They are most commonly prescribed to people with presbyopia who also require a correction for myopia, hypermetropia, and/or astigmatism.
Etymology
The term "bifocal" is derived from Latin, with "bi-" meaning two and "focal" meaning of the focus. It was first used in the late 18th century to describe these types of eyeglasses.
History
Bifocals were invented by Benjamin Franklin in the 1780s. As a prominent statesman, inventor, and polymath, Franklin was often troubled by the need to switch between two pairs of glasses. He solved this problem by cutting two lenses in half and combining them into a single frame.
Design
Bifocals typically have a larger area for distance vision at the top and a smaller area for near vision at the bottom. The two areas are separated by a visible line. The upper part is for viewing things at a distance, and the lower part is for reading and other close-up work.
Types
There are several types of bifocals, including flat-top (or D-segment), round-segment, executive (or Franklin), and progressive (or no-line) bifocals. Each type has its own advantages and disadvantages, and the choice often depends on the wearer's specific vision needs and lifestyle.
Related Terms
- Trifocals: Eyeglasses that have three distinct optical powers.
- Progressive lenses: Eyeglasses that have a smooth transition between different optical powers.
- Multifocal lenses: A general term for eyeglasses that have more than one optical power.
External links
- Medical encyclopedia article on Bifocals
- Wikipedia's article - Bifocals
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