What Are Bifocal & Trifocal Glasses?

Bifocal and trifocal glasses are spectacles that help with presbyopia, a reading vision correction that occurs is people around the age of 40. Bifocal and trifocal lenses allow you to see both in the distance and up close with one pair of glasses.

History

• Bifocals were invented by Benjamin Franklin in 1784. He combined a pair of his distance glasses and reading glasses by cutting the lenses in half and putting them together in one frame.

Function

• With bifocal and trifocal glasses, you look through the top part of your glasses to see into the distance and as you look down through the bifocal and trifocal areas, you can see things that are closer.

Features

• Bifocals have two prescription powers, one for far away and one for up close, usually 18 inches or closer. Trifocals have three powers, the two used in a bifocal and a third, used for an intermediate range, most often your arm's length. Trifocals are useful for reading things like the dashboard of your car or a computer monitor.

Bifocals and Trifocals with Lines

• Bifocal and trifocal glasses are available with lines and without. Lined bifocals and trifocals come in different sizes, called segments and are between 25 and 35 millimeters. The wider the segment, the bigger reading area.

No-line Bifocals and Trifocals

• The lenses without lines are called progressive bifocals but they are actually more like trifocals. Progressive lenses have a prescription power for nearer vision that gets stronger as it moves down the lens. No-lines let you read at many different focal points, not just one or two like lined bifocals and trifocals.