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Progressive Addition Lenses 137

Fig. 11.14: Progressive addition lens design

results in an inevitable consequence in the form of surface astigmatism at
the temporal and nasal side. Surface astigmatism produces an unwanted
astigmatic error or cylinder error that can blur vision and limit the wearers
field of clear vision. Therefore, this astigmatic error essentially serves as a
boundary for the various zones on the progressive lens surface. Unwanted
cylinder which is the consequence of the lens design is influenced by:

Add Power

The amount of astigmatism will be directly proportional to the add power
of the lens. A + 2.00D addition, for example, will generally produce twice
as much cylinder error as + 1.00 D addition.

Length of the Progressive Corridor
Shorter corridors produce more rapid power change and higher levels of
astigmatism, but reduce the eye movement required to reach the near zone.
Larger corridors provide more subtle power changes and lower levels of
astigmatism, but increase the eye movement required to reach the near
zone of the lens.

Width of Distance and Near Zone
Wider distance and near zones confine the astigmatism to smaller regions
of the lens surface, which produces higher levels of astigmatism. However,
wider zones do provide larger areas of clear vision. The location of the
near zone, which is offset nasally to account for convergence in down gaze,
is critical to wearer’s comfort and its position varies between designs.