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60 Ophthalmic Lenses

Influencing Factor
1. Applanatic curvature of the cornea reduces marginal astigmatism as
well as spherical aberration.
2. The spherical surface of the retina also results in minimizing the effect
of marginal aberration.
3. Finally, the astigmatic image falls on the peripheral retina , which has
relatively poor resolving power compared to the macular area. Visual
appreciation of the astigmatic image is, therefore, limited.

Clinical Application
1. Pantoscopic tilt in the spectacle creates astigmatic aberration which be-
comes more substantial with higher power and greater amount of tilt.
2. Myope sometimes tilts their glasses forward to gain more minus power,
indicating a need for stronger prescription. Tilting forward induces both
minus spherical and minus cylinder.
3. Oblique astigmatism aberration can also be a problem when objective
measure of the refractive error of an eye is attempted. This is especially
important when retinoscopy is performed on a patient who does not
maintain fixation, as commonly seen in small children.

COMA
Coma is a wide beam aberration and is somewhat similar to spherical
aberration. It is applied to oblique rays coming from points not lying on
the principal axis. Oblique rays passing through the periphery of the lens
are deviated more than the central rays and come to focus near the principal
axis. The result is unequal magnification of the image formed by the
different zones of the lens. The composite image is not circular , but
elongated like a comet and coma.

Fig. 6.7: Peripheral rays form larger image than paraxial rays