Bifocals

File:Bifoklabrille fcm.jpg

Bifocals are eyeglasses with two distinct optical powers. Bifocals are most commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.

History

Benjamin Franklin is generally credited with the invention of bifocals. Serious historians have from time to time produced evidence to suggest that others may have preceded him in the invention; however, a correspondence between George Whatley, a London philanthropist and friend of Franklin, and John Fenno, editor of The Gazette of the United States, suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought.

Since many inventions are developed independently by more than one person, it is possible that the invention of bifocals may have been such a case. Nonetheless, Benjamin Franklin is certainly among the first to wear bifocal lenses, and Franklin's letters of correspondence suggest that he invented them independently, regardless of whether or not he was the first to invent them.

John Isaac Hawkins, the inventor of trifocal lenses, coined the term bifocals in 1824 and credited Dr. Franklin.

Construction

Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908.

Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes. The most popular is the D-segment, 28 mm wide. While the D-segment bifocal offers superior optics, an increasing number of people opt for progressive bifocal lenses.

Problems

Bifocals can cause headaches and even dizziness in some users. Acclimation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes. Computer monitors are generally placed directly in front of users and can lead to muscle fatigue due to the unusual angle and constant movement of the head. This trouble is mitigated by the use of trifocal lenses or by the use of monofocal lenses for computer use.

Future

Research continues in an attempt to eliminate the limited field of vision in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers have constructed such a lens using a liquid crystal layer sandwiched between two glass substrates.

Bifocals in the animal world

The diving beetle Thermonectus marmoratus has recently become notable when it was discovered that its aquatic larval stage has been found to have used in its principal eyes two retinas and two distinct focal planes that are substantially separated, in the manner of bifocals to switch their vision from up-close to distance, for easy and efficient capture of their prey, mostly mosquito larvae. This is the first ever recorded use of bifocal technology in the animal world.

Sources

• Letocha, Charles E., M.D. (1990). "The Invention and Early Manufacture of Bifocals". Survey of Ophthalmology. 35 (3): 226–235. doi:10.1016/0039-6257(90)90092-A. PMID 2274850. CS1 maint: Multiple names: authors list (link)
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