Sometimes our vision fails us at two or even three distinct distances, especially as we age. Bifocal lenses—lenses with two distinct viewing areas—have traditionally been a reliable solution to such a dilemma. (A lens with three distinct viewing areas is called a trifocal.)
By distinct, we mean there are noticeable lines separating the two different fields of vision within a bifocal 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 are trying to see.
A farsighted person who also has trouble reading may be prescribed a pair of bifocal reading glasses, for example. The upper section of the lens would correct difficulties seeing objects at distance, and the lower section would assist in reading.