It’s worth noting that there is no direct relationship between indexes of refraction (at the top of the post) and corrective diopters (at the end). The closest you come is the lens-makers formula which tells you how to take those two numbers and build a suitable lens.

@dmckee: Higher index of refraction allows for less thickness change to get the same diopter. For negative diopter, the lens will be thin in the middle and thick on the outside. -7 is extreme (to the point where I wonder if it is correct), so the edges of the lenses will be significantly thicker than the center. Frames have edge thickness limits they can handle well. At -2 diopter, for example, this might not have been a problem.