Bifocals

Bifocals are eyeglasses that have two sets of corrective lenses for people who suffer from both myopia (nearsightedness) and hypermetropia (farsightedness).

This form of corrective lens set was invented by Benjamin Franklin who needed two sets of glasses to deal with each aspect of the above kind of impaired vision.

This proved cumbersome, but the solution presented itself when he realized that when a person views something at close range, they usually look down and the reverse is true when they view at a distance. Taking advantage of that fact, he designed bifocals with the lenses for close viewing in the lower half of the frame and the distance viewing lenses on the upper. Originally he simply cut each lens in half and combined the separate halves together in the frame, but now most bifocals are made by first creating a set of lenses for nearsightedness and then grinding a small section of that lens for farsightedness.

With the advent of computers, bifocal users have experienced some problems. Although most printed reading materials are easily viewed with bifocals, computer monitors are generally placed directly in front of users, but still close enough to require corrective lenses, requiring bifocal wearers to tilt their heads up to view the screen.