[Centering of eyeglasses with special reference to aspheric single-power lenses]

In the positive power range, aspheric single-power lenses have clear advantages for the spectacle wearer over highly curved spherical lenses in terms of cosmetic appearance and comfort in wear. Correspondingly flat spherical lenses would not be acceptable, due to their dramatically poorer image reproduction properties. Only with an aspheric surface design is it possible to combine a flat lens with optimum image reproduction properties. In order to realize these advantages of aspheric surfaces, the lenses have to be centered in accordance with the ocular pivot point. Several examples are given to illustrate the effects of incorrect centering on the image reproduction properties of both spherical and aspheric single-power lenses. The negative effect of incorrect centering in accordance with the ocular pivot point is clearly far greater with the mathematically much more complex aspheric surfaces than with highly curved spherical lenses. Although the image reproduction quality of the aspheric lens is superior--at least on paper--when it is correctly centered, it is equally true that in practice these lenses quickly prove inferior if incorrectly fitted.