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46 Ophthalmic Lenses

reduces from the center to the edge, the outcome of this flattening is the
reduction of prismatic effect in comparison with the same point on a
spherical lens. The flattening is not the primary effect of aspheric lenses
but is the side effect of the surface geometry.

Worked Prism for Decentration
Worked prisms work in exactly the same way, but creates the opposite
effect. Here, we are looking to avoid creating unwanted prismatic effect at
the center point. When the uncut is not available in the required diameter,
working a prism on the lens will move the optical centre across the uncut
lens creating a greater effective diameter.

Fig. 5.7: Worked prism

Prescribed Prism
The prisms may be prescribed to create a desired effect upon the binocular
balance of the eyes; this may be to reduce the visual stress to a tolerable
level where a phoria is involved or to relieve a convergence insufficiency.

Differential Prism at Near
In case of anisometropic population looking away from the optical center
create a loss of contrast and reduction in visual acuity which causes a
breakdown in binocular vision and diplopia due to the induced differential
prismatic effect. For a single vision spectacle wearer, this can be kept in
check by looking through the optical center, but for the multifocal wearer,
this differential prism must be tackled. In a multifocal lens, the zone that is
used to read lies below the distance visual zone. The center of the near
visual zone, is usually taken to lie somewhere around 8 mm below DVP
and 2.5 mm inwards. Assuming that the impact of pantoscopic tilt has
been taken care of, the prismatic effects generally differ at the visual points.
The difference in the prismatic effects between the eyes is called differential
prism and requires the eyes to rotate by different amounts to maintain
binocular single vision of the object of regard. Remember eyes can cope
with quite big horizontal differential prismatic effects, but the great
discomfort arises if the eyes encounter vertical prism, as it quickly produces