‘Necessity is the Mother of Invention’
By Tina on July 14, 2010Add CommentsThere is a popular belief that only after aging we should move to bifocal lenses, but this is not necessarily true as many of our younger generation have combination eye sight problems.
Bifocal lenses are corrective lenses, commonly prescribed for people with presbyopia, and have two different power areas to provide you with your visual acuity; a distance area and a reading area. However, if correction at intermediate distances is also needed, then trifocals and progressive lenses, also known as varifocal lenses, may be used. This being that a Bifocal lens has no intermediate viewing zone as there is in a varifocal Lens. Bifocal lenses have the fine ‘line’ in the middle of the lens to separate the distance and reading portions.
The bifocal form of corrective lens set were made popular in the late eighteenth century by Benjamin Franklin when he became frustrated at needing two sets of spectacles to deal with each aspect of his impaired vision, namely his distance and reading area. It is, however, likely that optical craftsmen in London had developed the split lens some years earlier – the design of lenses Franklin commissioned from them came as no surprise.
Originally the bifocal lenses were simply cut in half and combined together in the rim of the frame, bearing in mind the fact that when a person views something at close distance, they usually look down and the opposite is true when they view at a further distance. The early bifocal lenses were designed with the lenses for close viewing in the lower half of the frame and the distance viewing lenses on the upper. Later designs involved the cementing of a bifocal segment onto a larger lens, but now most bifocals are made by fusing a small half-moon shaped reading segment into the lens.
Today 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 which is 28 mm wide.
Since the new age 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.
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