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10

Bifocal Lenses

Eyes age along with rest of us and vision stealing problem becomes more
troublesome beginning in the middle age. Usually around the age of
40 years, when a person is found squinting at the newsprint held at the
arm’s length, he has achieved the middle age milestone: the arrival of
presbyopia. A presbyopic subject requires a separate correction for distance
and near vision, the two prescriptions may be provided as one pair of
spectacle in the form of a bifocal lens. A bifocal lens is defined as having
two portions of different focal power. The area of the lens used for distance
vision is called Distance portion or DP and the area used for near vision is
called the Near Portion or Reading Portion or NP. The larger of these two
portions is referred to as the main lens (the exception being the up curve
bifocal).
The first recorded mention of bifocal spectacle lens is a letter written by
Benjamin Franklin in 1784 in which he described a pair of spectacles
incorporating such lenses. These were made by the relatively crude method
of splitting a distance and a near lens, then mounting the top half of the
distance and the bottom half of the near together and were fitted edge-to-
edge in a single rim as shown in Figure 10.1.

Fig. 10.1: Split bifocal (Franklin design)