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144 Ophthalmic Lenses

Fig. 11.21: Astigmatic contour plot of a PAL (+ 2.50 add)

unavoidable by progressive addition lens design process, it is legitimate
for the lens designers to map the location and degree of cylinder with each
design interaction. “Soft” design have fewer zones of induced cylinder,
represented by fewer lines on the isocylinder contour plots, located further
apart than in “hard” design. Hard designs, on the other hand, have a greater
number of lines located closer together. To the untrained observer, the hard
design may appear superior because of the longer areas free of induced
cylinder. However, clinical experience has shown that wearers perceive
soft design as more comfortable in dynamic and peripheral vision as they
do not have a high concentration of cylinder in areas critical to peripheral
and dynamic vision, as in the case with hard designs.
Lens designers also use isocylinder contour plots to depict the presence
or absence of asymmetry in the progressive addition lens design by
comparing the nasal and temporal aspects on either side of the oblique
path of power progression. An asymmetric design produces identical optical
characteristics on both sides and thus, sphere power cylinder, and vertical
prism are almost identical for both eyes in any direction of gaze, promoting
binocularity and comfort. A symmetric design employs a single design
that is rotated about 10 degrees in one direction to create a “right” lens and
about 10 degrees in the opposite direction to create a left lens. In a hybrid
of these two approaches, referred to as dissymmetric design, separate
designs are used for right and left lenses. However, since the entire
unwanted cylinder has been pushed from the temporal to the nasal side,
different optical characteristics are present on either side of the lens.