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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
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