Sometimes our vision fails us at two or even three distinct distances, especially as we age. Trifocal lenses—lenses with three distinct viewing areas—have traditionally been a reliable solution to such a dilemma. (A lens with two distinct viewing areas is called a bifocal.)
By distinct, we mean there are noticeable lines separating the three different fields of vision within a trifocal lens surface. A slight adjustment to the angle of the head allows wearers to choose which lens area to look through based on the distance of the object they’re trying to see.
A farsighted person who also has trouble reading and seeing at arms distance may be prescribed a pair of trifocal reading glasses, for example. The upper section of the lens would correct difficulties seeing objects at distance, the middle of the lens is for a computer screen and the lower section would assist in reading.