title: Spectacle correction for aphakia. The prismatic effect.
abstract: 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.
mesh_terms: D001036:Aphakia, Postcataract / Q000628:therapy*; D005133:Eye Movements; D005139:Eyeglasses / Q000592:standards*; D006801:Humans; D055095:Optics and Photonics