From Wikipedia, the free encyclopedia
Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.
Bifocals with separate lenses
Benjamin Franklin is generally credited with the invention of bifocals. Historians have produced some evidence to suggest that others may have come before him in the invention; however, a correspondence between George Whatley 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 was among the first to wear bifocal lenses, and Franklin’s letters of correspondence suggest that he invented them independently, regardless of whether 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.
In 1955, Irving Rips of Younger Optics created the first seamless or “invisible” bifocal, a precursor to all progressive lenses.
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.
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 straight 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 users.
In an interesting legal case reported in the UK in 1969, the plaintiff’s ability to use bifocals was impaired by accident.
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 aquatic larval stage of the diving beetle Thermonectus marmoratus has, 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.