For high power single-vision lenses, significant improvement can be achieved with freeform lenses, even with spheric blanks. The following plots describe the performance for a surface optimized for a +13 Diopter prescription with CR-39 and  a spheric blank.

As the first plot shows, the aspheric back surface is most strongly curved for a plus lens, resulting in a thinner design.

The next two plots show the improvement in both sphere and cylinder error across the gaze angles. The constant spherical equivalent means there is a constant magnification and hence the image is not distorted.

Aspheric Back Surface Designs

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