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Lens Aberrations 63
from plus lens. Distortion is of minor importance in ophthalmic lens
designing. It is a problem mainly of high power lens. It can be minimized
by using steep back surface.
SUMMARY
In designing a spectacle lens, the designer has very limited degree of
freedom. Practical conditions specify lens materials, safety considerations
fix lens thickness, and fashion dictates lens position before the face, weight
and cosmesis mean only two lens surfaces may be used. With so few degrees
of freedom, few aberrations may be corrected. The wide angle spherical
and cometic aberrations are of no importance since the entrance pupil of
the eye is so small. The visual system adapts quickly to distortion, so the
additional aberration of spectacle lens may be ignored. Thus, the spectacle
lens designer is left only with the job of correcting astigmatism and
curvature of field within tolerable limits so as to produce maximum
dynamic field of vision for the patients.
