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112 Ophthalmic Lenses

Fig. 10.13: Section through the common optical centres of distance and near
areas of an E-style bifocal in case of minus prescription

Another problem associated with the type of bifocal lens is the reflection
from the top edge of the straight top bifocal which can be annoying to the
patient. Reflection from this edge can often be seen by others, making the
segment more obvious to others.

Fig. 10.14: Reflection from the top edge of straight top bifocals can be annoying
to the patient. It can also be seen by others

OPTICAL CHARACTERISTICS OF BIFOCAL LENSES

Optical characteristics of bifocal lenses vary widely from one bifocal style
to other, so they should be taken into consideration when selecting a bifocal
segment style for a given patient.

IMAGE JUMP

When looking from distance portion to the near portion of a bifocal lens,
the sudden change in the prismatic effect because of the introduction of
the base down prism by the segment causes the world to “jump”. This
displacement of the patient’s visual world can be very disturbing and often
dangerous particularly with steps and kerbs. It has both vertical and
horizontal element. Jump occurs all around the edge of the bifocal segment,
although it is at the top part which is most frequently noticed.