Spectacle correction for aphakia. The prismatic effect.

Because spectacle lenses are fixed in position relative to the eyes, the visual axes during convergence pass nasal to the optical centers, and create a prismatic effect. The eye rotates less than 25 degrees before the head is turned; therefore for central vision, only an area of the lens about 13 mm wide is used. A reading addition of +6.00 D is probably the maximal power bifocal addition that can be prescribed for comfortable binocular vision in a high plus correction. The distance correction produces a base-out effect of at least 4 delta for each eye at the reading distance, even though the bifocal segments are adequately decentered.