Source:allaboutvision
Bifocals and trifocals have been around for many years to help people over age 40 deal with the normal age-related loss of near vision calledpresbyopia.
Bifocal and trifocal lenses contain two lens powers (bifocals) or three (trifocals) to help restore a fuller range of vision and, in particular, to restore reading vision lost to presbyopia.