Bifocal glasses are generally prescription glasses with lenses which have two different optical powers. These bifocal glasses are prescribed to people suffering from presbyopia and who also require a correction for myopia, hyperopia and/or astigmatism.
Though various historians have produced evidence from time to time suggest otherwise, Benjamin Franklin is generally considered to be the inventor of bifocals. But historical articles including correspondence with George Whatley (a London philanthropist) and John Fenno (editor of The Gazette of the United States of America), convincingly proves that Benjamin Franklin invented bifocals. Since many inventions are developed independently, and by more than one person, many historians suggest that it might have been the case with the invention of bifocal glasses. But without doubt, Benjamin Franklin was among the first to wear bifocal glasses, whether he invented it or not. In fact, John Issac Hawkins, who was the inventor of trifocal lenses, coined the word “bifocals” in the year 1824 and he credited Benjamin Franklin.
In case of designing bifocal glasses the original ones were designed with the most convex lenses in the lower half of the frame and the least convex lenses on the upper. The lower part of the frame was used for viewing nearby objects while the upper half for the distance viewing. Till the beginning of the 20th century, the lenses were made using two separate ones being cut in half and then combined together in the rim of the frame. This process of mounting two half lenses into a single frame obviously resulted in numerous complications and many users found such spectacles much fragile. By the end of the 19th century, Louis de Wecker managed to develop a method for fusing the sections of the lenses together. Later, Dr. John L. Borsch Jr. patented this method in 1908.
Now-a-days, bifocal lenses are mostly crafted by molding a reading segment, used for near vision, into the primary lenses. Since these lenses are available with the reading segments in diverse shapes and sizes, the choices for buyers have also increased. One of the most popular ones is the D-segment which is generally 28 mm wide. Though these D-segment bifocal lenses offer better viewing qualities, an increasing number of users are going for progressive bifocal lenses.
Bifocal glasses are known to cause headaches and even dizziness in some users. Many users take time to adjust to the small field of view which is offered by the reading segment of the bifocals. This is usually done by user learning to move the head or the reading materials rather than the eyes.