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116 Ophthalmic Lenses

The total prismatic effect at the NVP is the sum of 2.4Δ BU and 2.8Δ BD,
which is 0.4Δ BD.
Now, consider a case of D-shape bifocal as shown in Figure 10.19. In
this case D shape segment size of 28 x 19 mm has been prescribed. If we
assume that NVP is located in exactly the same position 8 mm below the
O and 5 mm below the segment top. The optical centre of the segment O s
D
now coincides with the NVP, so segment adds no prism at this point. So
the net prismatic effect would be only 2.4Δ BU, what is exerted by distance
portion. This is an important advantage of the shaped bifocal.

Fig. 10.19: D-shaped segment of 28 × 19. Os is the optical centre of segment
which lies 5 mm below the segment top and coincides with NVP

Fig. 10.20: E-style bifocal. The segment exerts base up prism at the NVP

In case of E-style bifocal, the optical centre of the segment is positioned
on the dividing line which is above the NVP and since the power of the
segment is positive, the segment will exert base up prism. If we assume
that NVP is located at the same position, i.e. 5 mm below the optical centre
of the segment or the segment top, the prismatic effect would be:
P = 0.5 x 2.00
= 1.0Δ BU