The American Optometric Association has recomm...
For many people, different lenses are needed for seeing at different distances. Bifocal lenses allow the wearer to look through two areas of the lens. One area focuses on distant objects. The other is used for reading. A little-known fact is that bifocals were invented by Benjamin Franklin, and his style of bifocals are still available today.
Most of the time the “reading” area is smaller, shaped like a sideways “D”, and found in the lower hemisphere of the lens. These bifocals are called line bifocals or flat-tops. If you are focusing on distant objects, you look through the top half of the lenses. To read a book, magazine, or newspaper, you look through the “reading” area. The Franklin style lenses are less common, and are split horizontally down the middle of each lens. One thing that is difficult about using bifocals is dealing with the line between the two vision areas. Fortunately, recent technologies have developed a new type of lens, called the no-line, or progressive, lens.
