The importance of base curve in the design of minus iseikonic lenses.

PURPOSE: A widely advocated approach to increasing spectacle magnification in minus lenses is to increase center thickness and reduce vertex distance. Conventionally, a steeper base curve is to be avoided because of the accompanying increased vertex distance in a minus lens and the reduction of spectacle magnification that results. This approach works when small amounts of spectacle magnification are needed. However, when large amounts are needed for high minus lenses with flat base curves, the conventional approach does not provide sufficient magnification. Facial features such as eyelash length limit the contribution of reducing vertex distance, and the flat base curves limit the efficacy of increasing center thickness. Steeper base curves are required to increase the optical "leverage" of increasing center thickness.

METHODS: Through use of an example, a steeper base curve is shown to increase the spectacle magnification of a high minus lens. The gain in spectacle magnification is preserved by maintaining a nearly constant vertex distance through bevel location.

RESULTS: In the example, changing the base curve of a -6.50-D lens by +4.50 D while increasing the center thickness by 4.1 mm resulted in a +4.4% increase in spectacle magnification. The lens bevel was kept as close as possible to the front surface, allowing the increased center thickness to keep the back vertex at essentially a constant vertex distance in the presence of the steeper base curve. Without the steeper base curve, it was not possible to attain the needed spectacle magnification.

CONCLUSIONS: When large amounts of spectacle magnification are needed for high minus lenses, both steeper base curves and increased center thicknesses are necessary. The reduction in spectacle magnification from increased vertex distance that is expected with steeper base curves can be neutralized by keeping the bevel close to the front surface on the thicker lens.