Marginal aberrations of different bifocal-lens designs.

Marginal astigmatism and longitudinal chromatic aberration were measured (with a modified Vertometer) for +2-D bifocal lenses of different designs, base curves, and major-lens powers. At a reading level 20 deg below the pole of the major lens, deterioration of the optical image occurred and was created by the interaction of marginal astigmatism (tangential error) and lateral chromatic aberration (prismatic dispersion). Suggestions for selecting an optimal bifocal-lens design for different types of refractice error are presented.