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94 Ophthalmic Lenses

ophthalmic lens. When the rays of light strike the periphery of the lens
obliquely, two focal lines are produced from each single object point. The
dioptric difference between these two focal lines is known as the astigmatic
error of the lens. The effect of this aberration is the reduction in wearer’s
field of clear vision through the lens. For conventional lens, front surface
base curve selection is one of the primary tools used to reduce the lens
aberrations. Flatter base curves produce cosmetically appealing lenses that
are flatter, thinner and lighter in weight. They are easily retained in the
frames as well. However, flattening the base curves introduces significant
aberrations in the periphery of the lens as shown in Figure 9.2.

Fig. 9.2: A comparison between steeper and flatter lenses. The steeper lens
provides better off- axis optics, while the flatter lens provides better cosmetics

So it is established that while the best form lens gives the least oblique
astigmatism, the flat lens gives the least bulbous appearance. The solution,
therefore, is one of the many aspheric design lenses available in the market
today.

ASPHERIC LENS DESIGN

Three dimensional aspheric surfaces are produced by rotating non- circular
curves about an axis of symmetry. The central curvature or vertex curvature
of an aspheric surface is nearly spherical which is the front curve value
utilized for lens power and surfacing calculations. Away from the vertex
curvature, the amount of surface astigmatism smoothly increases. The rate
of increase in surface astigmatism depends upon the degree or type of
asphericity.