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108 Ophthalmic Lenses

Fig.10.7: Cemented bifocal

FRANKLIN BIFOCAL
Also known as split bifocal is made of two separate lenses held together by
the frame. They are almost invariably an E-style bifocal, although very
rarely they are made as very large segment diameter in down curve forms.
They sometimes have their two sections bonded together and they can be
made in either glass or plastic materials. Unfortunately, dirt tends to
accumulate in the joint between the lenses, however well made they are
and no matter how careful the subject is in cleaning them.

BIFOCAL SEGMENT SHAPES
Segments vary from tiny “B segment” to “E-style” in which the whole
bottom half of the lens is used for near portion. Historically, there were
huge number of segment shapes with minor variations and corresponding
number of descriptions used for them. But now the apparent choice has
reduced, so that a small number of terms can be used to cover all multifocals.
The most popular bifocal shape at present in common use is probably D-
segment, E-style and round segment bifocal. B-segment and C-segment
are relatively rare, while any other shape is almost unheard of.

ROUND SHAPED BIFOCAL
Round segment bifocal have a segment with a dividing line that is a single
circular arc which is least visible compared to the other bifocals. When
placed at the bottom half of the lens, they are often called “down curve”
bifocal. The centre of the segment in round shape bifocal fall much below
from the top, so they must be fitted a little higher than the other bifocals.