Eikonic lens design for minus prescriptions.

Designing size lenses for aniseikonia correction usually involves considering only the shape factor of the total spectacle magnification. However, for minus prescriptions, an increase in the shape magnification of a lens does not always result in total lens magnification. As the front curve of a lens is steepened (which increases the shape magnification), the sagittal depth of the lens is increased, which effectively increases the vertex distance. For a minus lens, an increased vertex distance means decreased power magnification; depending upon the lens power, the decrease in power magnification can totally offset the increase in shape magnification. The shape factor-power factor relationship of high minus size lenses is presented, along with general guidelines for proper design of size lenses.