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114 Ophthalmic Lenses

Fig. 10.16: Image jump at the top of round segment due to the segment alone is
the product of the addition power and the radius of the segment in centimeters. For
a shaped bifocal, say ‘D’ segment the prismatic effect at the top edge is the distance
of the top edge from the centre of the circle of which the segment is the part multiplied
by the addition

Fig. 10.17: Lateral jump in an E-style bifocal. Line at the optical centre is undeviated.
But vertical lines at A and B show a lateral displacement at the segment top
For example, if the reading addition is + 2.00D, the jump exerted by a
24 mm segment is 2.4 Δ BD in case of round segment bifocal.
But in case of D segment where segment size is, say, 28 x 19 and the
segment centre lies just 5 mm below the segment top, the jump will be
only 1.00Δ BD. Reduced jump is probably the one reason why shaped bifocal
have proved to be so popular.
To eliminate the jump effect in a bifocal lens, it is necessary to work the
segment in such a fashion that its optical centre coincides with the segment
top.

PRISMATIC EFFECT
When a wearer uses single vision lens and he gazes downward to read
through the lens, he encounters a prismatic effect. The prismatic effect will
change in the near visual zone when he wears a bifocal lens. While
determining the prismatic effect in the near portion, it is to be understood