What Are Bifocals & Why Are They Used?

Bifocals are glasses or contact lenses that help people see objects both near and far.

History

• People have been using magnification to help see for about 2,000 years, when polished clear rocks were called reading stones. According to the "Book of Optics," optical lenses were first used about 1,000 years ago, and Benjamin Franklin invented bifocals in the 1700s.

Types

• Bifocal glasses can be worn as lenses with obvious lines, or they can be "invisible" bifocals, also called no-lines or multifocals. There is usually a prescription on the top of the lens for distance vision and a reading prescription on the bottom for near.

  Bifocal contacts work similar to spectacle lenses.

Size

• Bifocals can be many sizes. They range from very small, 22 cm, to segments that go all the way across the bottom of a spectacle lens. Size usually depends on the use of the bifocal, the prescription and the size of the frame.

Function

• Bifocal lenses work by looking through the distance part of the lens when focusing on objects farther away, and looking down through the bifocal segment for close items. Bifocals with lines have two focal points, one for distance and one for near. No-line bifocals are less obvious. They also have many focal points. This allows you to read at many distances.

Considerations

• Most people start wearing bifocals around age 40 to 45, although sometimes they are prescribed for children, too. Some children have trouble with focus, and wearing bifocals helps their eyes learn to work better. Most children outgrow the need for bifocals.