Prism - thinning progressive lenses

A progressive lens produces a smooth increase in positive foca_or addition_ power from distance to near without any lines of demarcation. In order to provide this gradual change in power, the curvature of a convex progressive lens surface must become increasingly steeper toward the bottom of the lens (Figure 1). The addition power of a convex progressive lens surface is simply the result of the difference between the higher surface curvature in the lower region_or near zone_of the lens and the lower surface curvature in the upper region_or distance zone_of the lens. A concave progressive lens surface, on the other hand, must become increasingly flatter toward the bottom of the lens in order to produce an increase in addition power; this type of surface is often encountered with modern ‘freeform’ progressive lenses.