HT1808 - The Perfect Lens

We all want flawless, optically perfect lenses. What if I were to ask you for a quantitative measure of the sharpness you need? How sharp is sharp enough? Is sharpness a constant across all genres of images? Or is acceptable for one subject overkill for another? To complicate this further, this fails to take into account such variables as viewing distance and print size. Enough is as good as a feast.