Engineer, you can not equate prescription strength with unaided visual acuity. The diopter is a unit of measurement for Lenses, so that the guy who makes your specs is on the same page as the optician. It serves no other purpose. Visual acuity is measured on the eye chart. The JAA are on very weak ground scientifically on this one. There should be no Diopter limit, just correctable to 20/20 or 6/6 which ever you prefer. This concept is proven safe beyond all possible doubt from the experience of the FAA.