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110 Ophthalmic Lenses

Fig. 10.9: (A) Straight top D bifocal. (B) Curve top bifocal. (C) B segment bifocal.
The dot indicates the geometric centre of the segment

segments, although it is less than round segments. A modified shape of D
shape bifocal is C segments which are also available with a slight rounded
top instead of flat tops. The principle advantage of C segments over D
segments is probably in glazing and surfacing. Minor errors made in the
cylinder axis during surfacing can be glazed to the correct axis and they
can be far less noticeable. B segments are like a D bifocal with the bottom
chopped off. Losing the top of the circle, as with the D-shaped bifocal,
reduces the jump at the edge of the segment and losing the bottom gets rid
of the area which is potentially the worst optically and thereby allows a
clear view of the feet through the bottom.

E-STYLE BIFOCAL
E- Style Bifocal is one piece bifocal lens with two different curves ground
usually on the front surface. The optical centre of both the distance and the
near lens lie on the dividing line. It is, therefore, classified as monocentric
bifocal. The difference in curves between the distance and the near portion
of the lens produces a ledge that is most prominent towards the outer edge
of the lens. E-style bifocals, popularly known as Executive bifocal are
invariably solid bifocal. Most of optical centre of the distance and near
portion on the dividing line gets rid of vertical image jump.

Fig. 10.10: E-style bifocal. The cross shows the position of optical centre