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58 Ophthalmic Lenses

Correction
1. Spherical aberration may be reduced by occluding the periphery of the
lens such that only the paraxial zone is used.
2. Lens form may also be adjusted to reduce spherical aberration. Aplanatic
surface where periphery curve is less than the central curvature may
be used.

Ocular Spherical Aberration
Refractive power of ocular structure is greater for peripheral rays than
paraxial rays. The effect of spherical aberration on retinal image is a
symmetrical blur like defocus which is reduced by the size of pupil.

Influencing Factor
The effect of spherical aberration in the human eyes is reduced by several
factors:
1. The anterior corneal surface is flatter at the periphery than at its centre,
and therefore acts as an applanatic surface.
2. The iris acts as a stop to reduce spherical aberration. The impairment of
visual acuity that occurs when pupil is dilated is almost entirely due to
spherical aberration.
3. The nucleus of the lens of the eye has a higher refractive index than the
lens cortex. Hence, the axial zone of the lens has greater refractive power
than the periphery.
4. Finally, the retinal cones are more sensitive to light which enters the
eye paraxially than the light which enters obliquely through peripheral
cornea. This directional sensitivity of the cone receptors limits the visual
effects of the residual spherical aberration in the eye.

Clinical Application
Spherical aberration accounts for much of the phenomenon known as
“Night Myopia”. At low light levels, the pupil enlarges and allows more
peripheral rays to enter the eye. The peripheral rays are focused anterior to
the retina, rendering the eye relatively myopic in the lower light levels.
The typical amount of night myopia is about 0.5D, but it can be as large as
1.25D.

MARGINAL ASTIGMATISM

Marginal astigmatism or oblique astigmatism aberration is a small angle
aberration. When a narrow beam of light enters obliquely to lens axis of a
spherical lens, the refracted rays become astigmatic. The emerging rays,
instead of uniting in a single image point, form two foci at right angles to