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62 Ophthalmic Lenses
Correction
Curvature of field affects peripheral vision. Curvature of field is minimized
with corrected curve design base curvatures. As a rule if marginal aberration
is corrected, the effect of curvature of field is also reduced. Good correction
of astigmatism results in small curvature of field.
Ocular Curvature of Field
In the human eye the curvature of the retina compensates for the effect of
curvature of field. So this is an advantageous aberration in the human eye.
DISTORTION
Distortion is another aberration of thick lenses. In distortion the object is
sharply imaged but does not retain its shape. There are two types of
distortions resulting from lateral magnification of the image that results in
a lateral displacement of the image:
Fig. 6.9: Pin cushion with plus lens and barrel distortion with minus lens
Barrel Distortion
Barrel distortion is produced in minus power lens where the rays in the
centre are more magnified than the rays farther off-axis. This is due to
minification of corners of a square grid more from minus lens.
Pincushion Distortion
Pincushion effect is produced in plus lens where the central rays are less
magnified. This is due the magnification of corners of a square object more