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Bifocal Lenses 111

To locate the segment centre, hold the lens 4 to 5 inches away from a
straight line and move the lens from left to right, the straight line will break
as it passes from the upper portion of the lens to the segment. At the segment
centre point, the straight line will have the continuity as shown in the
Figure 10.11.

Fig. 10.11: The segment centre of E-style bifocal is at the point where the
straight vertical line will have continuity

The big advantage of E-style bifocal is that the entire bottom half of the
lens is for near vision. Thickness can be a problem in hypermetropic
prescription. Because of the fact that both distance and near optical centre
fall on the dividing line, the thickness of the finished lens is largely
controlled by the resultant near prescription rather than the distance
prescription. Hence, + 3.00D with + 3.00D addition would have a centre
thickness equivalent to + 6.00D, i.e. for a plus prescription the centre
thickness is equivalent to the reading addition plus the distance power.
The edge thickness can be comparable to – 3.00D, i.e. the difference between
distance prescription and the resultant near prescription. Prism thinning
can reduce but can never eliminate the effect.

Fig. 10.12: Section through the common optical centres of distance and near
areas of an E-style bifocal in case of plus prescription
Contrarily, in minus lens the centre thickness is affected by less power
than the distance prescription, i.e. if the distance prescription is – 4.00D
and the near addition is + 2.00D, then the sum of distance power and the
near addition is – 2.00D. So, the centre thickness can be comparable to
– 2.00D. Prism thinning can improve the appearance of the top edge.