Eyewear
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Bifocals
Flat-top Bifocals
The portion of the lens used for near point viewing is usually not very large
and is located near the lower edge. This type of lens is satisfactory for
occasional near-point work or near-point work that is usually in, or may
be easily moved to, the lower part of the field of view. In this case, however,
lateral viewing of near point objects is restricted.
Full-Width Bifocals
The lens is divided into two parts: an upper part for distant viewing and a
lower part for near point viewing. The straight line dividing the two parts
may be above, at or below the normal line of sight for distant objects, and
both parts provide a wide field of view.
Progressive-Addition Bifocals
For people who are distracted by the dividing line in full-width bifocal, the
progressive-addition bifocal smooths out the division between the upper and
lower parts of the lens. That smoothing, however, which occupies a band a
few millimeters wide, causes some distortion that may itself be distracting.
Trifocals
Intermediate segments may be added to the flat-top and fullwidth bifocal to
make them trifocal lenses.
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