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22 Ophthalmic Lenses

Fig.2.9: Lens form comparisons
within a range of + or – 1.00D, then the base curve should generally be
duplicated in the new pair of lenses. If the prescription is noticeably
different, a slight adjustment in the base curve may be suggested (Fig. 2.9).
While selecting a new base curve for a new wearer or adjusting the
base curve of an existing wearer, it is important to understand how the
selection of a curve will affect the finished product. Under ideal
circumstances, a spectacle lens would reproduce a perfect image on the
retina. However, this is not always the case. The lens designer has to deal
with varieties of aberrations which prevents perfect image through the
lens. There are six types of aberrations that prevent perfect imaging through
the spectacle lens:
1. Chromatic aberration
2. Spherical aberration
3. Coma
4. Marginal aberration
5. Curvature of field
6. Distortion
Chromatic aberration is caused because shorter and higher energy
wavelengths are bent more readily and to a greater degree than the longer
and lower energy wavelengths. The lens designer must deal with this kind
of aberration by selecting a lens material with low dispersive value.
Spherical aberration results in a slight blurring of the image due to lenses
having the tendency to focus the light rays from the periphery sooner than
the rays passing through the central portion of the lens. This results in
separate foci which prevent a sharp clear image on the retina. But in general
spherical aberration is not a great problem to the lens designer as the small
pupillary size is effective in keeping out its effect.