The folks at DxOMark have a new measurement, Perceptual Megapixel. This purports to boil down the information from modulation transfer function (MTF) graphs into a number. It proves that a camera with an enormous number of pixels isn’t all that useful unless you have an amazingly high quality lens and, probably, have locked the camera/lens down to a tripod. Folks who have a 20 megapixel camera and a $200 kit lens might be better off simply capturing at 6-10 megapixels. See, for example, a test of a Nikon superzoom lens in which 6 perceptual megapixels was all that could be extracted (third party superzooms came in at 4 or 5 megapixels). This is the resolution that Kodak selected for its consumer PhotoCD system back in the early 1990s.
Summary: the electrical engineers have pushed sensor resolution far beyond the capability of any optics that ordinary consumers are willing to purchase and carry.
[Separately, one of the first tests done using this metric shows that the $900 Sigma 35/1.4 lens dramatically outperforms both Canon's $1300 equivalent and an $1800+ Zeiss manual focus lens. (The failure of prestige names to dominate objective tests is not new. I remember years ago a European photo magazine did optical bench tests of 50mm lenses and concluded that the $100 to $200 Canon and Nikon 50/1.8 lenses outperformed Zeiss and Leica lenses costing up to $2000. If memory serves, the magazine selected the Nikon 50/1.8 as the best lens overall, considering the balance of sharpness and distortion.)]