Bifocals
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Bifocals are eyeglasses that have corrective lenses containing two different lens powers. They are most commonly prescribed for people with presbyopia, and who also require a distance correction for myopia or hyperopia (and often astigmatism). If correction at intermediate distances is also needed, trifocals and progressive lenses may be used. The bifocal form of corrective lens set was popularised in the late eighteenth century by Benjamin Franklin who had grown frustrated at needing two sets of spectacles to deal with each aspect of the above kind of impaired vision. It is likely, however, that optical craftsmen in London had developed the split lens some years earlier as the design of lenses Franklin commissioned from them came as no surprise.
Taking advantage of that fact that when a person views something at close range, they usually look down and the reverse is true when they view at a distance, the early split 'Franklin' bifocals were designed with the lenses for close viewing in the lower half of the frame and the distance viewing lenses on the upper. Originally the lenses were simply cut in half and combined together in the rim of the frame. Later examples involved the cementing of a bifocal segment onto a larger lens, but now most bifocals are made by molding a small reading segment into the lens.
Bifocal lenses are available with the reading segments in a variety of shapes and widths. The most popular is the flat-top or D-shaped segment, 28 mm wide.
With the advent of computers, bifocal users have experienced some problems. Although most printed reading materials are easily viewed with bifocals, computer monitors are generally placed directly in front of users, but still close enough to require corrective lenses, requiring bifocal wearers to tilt their heads up to view the screen.
External Links
- Bifocals consumer review (http://www.1-contact-lenses-consumer-guide.com/bifocals.html) (article)es:Bifocal