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.