Although CR - 39 or 1.49 refractive index of lens can installed the high rx, it will cause with the coke bottle shape. It’s very funny. Of course, the optical companies have other ways to minimize the thickness of the lens. It is that the technicians cut and narrow the diameter of presbyopic lens to small shape, so that make customized cutting variable thin edge of lens. Due to comparing with the myopia lens, the presbyopic has thicker middle and thinner edge.