Interesting question. When talking about the ability of the human eye to differentiate lens differences, there are a few factors involved. Starting with single vision lenses, my understanding is that most people can differentiate about a 0.25 diopter difference, which is the difference between the various lenses the eye doctor uses in their machine to check your vision (phoropter
). I've read that some people can tell the difference at a 0.125 diopter level, but I find that usually the eye doctor can get down to two lense choices that look almost identical to me. I can't imagine trying to determine which is better at a 0.125 diopter difference.
Additionally, the glasses themselves are allowed some variance in production. I've read the allowed amount is 0.25 diopters, but I would guess that most eyeglasses are made more accurately than that.
So - based on the above discussion, it seems that the technology of single vision lenses is already able to surpass what the human eye can discern in most cases. Single vision lenses are generally not too horribly expensive, but you would pay more for high index (thinner) and polycarbonate (shatter resistance). The real increase in cost is when you get into progressive lenses. I remember reading some studies where manufacturers did compare the types of lenses, but it was an older publication. Essentially they did compare two of progressive lenses and they found one was preferred more often than the others. I'll see if I can dig up the study.
Bottom line (as best I can tell) - the lenses can be made even more accurate, yet the human eyes is not able to tell the difference.
One other aspect, though, is the high cost of progressive lenses, and whether that extra cost is worth it. I have yet to see a good study of progressive lenses, at least more than comparing two designs. This seems to be the most important question - are the expensive lenses worth the extra cost? Are the lenses actually better than the cheaper lenses?