Simple fact of the matter is a better lens will perform better on ALL sensors, 20mp, 30mp, or 50mp. The problem with DXO's tests is they quite simply don't give you a reasonable camera-agnostic basis from which to compare lenses.
Actually, they do. There is a way to extract the pure lens resolution from the data they used to publish (full MTF curves, not the nonsense they publish now).
Umm, no...sorry. The final image is a convoluted result...one could not extract a "pure" lens resolution...you could only approximate it. (For the very same reason one cannot perfectly extract noise from a noisy image...it is part of a convolution produced by a complex real-world system. Too much uncertainty and a loss of information prevents perfect noise removal.) A mathematically generated MTF that takes into account the real mathematical point spread function of the entire lens is really the only way to get any realistic idea of how a lens will actually perform. The moment that convolution is further convolved by a sensor, you lose the ability to "perfectly" (or purely) revert to the prior result...there is too much uncertainty and loss of information.
QuoteThe Nikon 500/4 performs "on par" (toung in cheek) with the Canon 500/4 solely because of the higher resolution sensor. That sort of tells you that the Canon lens is particularly good, because it is performing so well on a worse sensor...but you don't really have any exact way of comparing. You only get a "feeling" that it performs so well.
Why in the world would you want to know how a Canon compares to a Nikon without a body? For bragging rights? They tell you what is achievable with the current bodies on which the lens works, the way it is deigned to work. A better lens on one body will be better on future bodies as well.
One wouldn't, necessarily. But your missing the point. The point is to call out DXO's BS approach to performing lens tests. The point is to clearly note that those tests are "camera system" tests...they are neither lens tests nor sensor tests. I wouldn't go so far as to say that is 100% useless, but it is certainly biased the way DXO does it, and there is a suspiciously long-term bias towards a particular manufacturer by DXO. (Not just away from Canon, either...even the Sony lens, which actually has better transmission, should have scored better...but it was limited by a sensor!)