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.)
You are wrong on that. I am not saying that you can remove the AA filters/sensor blur from the image. I am saying that you can find (estimate, if you wish) the strength of the sensor blur. If you are interested in the math, go to my profile, click on the link, etc. Deconvolution is a very different process, very unstable but you do not need to deconvolute to estimate the effect of the sensor blur. You can get instability only if you use sensors with such a low resolution, that the lenses you want to compare look the same (and they are not).
The problem with all that is that even if you are going to get the pure lens resolution somehow, you still need to consider the blurring effect of a future sensor, and compute the combined resolution again. So my question stands: are you sure you know how to do that?
It doesn't matter what kind of sensor you have, low resolution, high resolution, or tomorrows resolution. A convolved result is a convolved result, and in this case stability (or the lack thereof) doesn't really apply like it might when trying to denoise or deblur. You are talking about reverse engineering the actual lens PSF from an image produced by a grid of spatially incongruent red, green, and blue pixels (likely covered by additional lenses (microlenses)), then further interpolated by software to produce the kind of RGB color pixels we see on a screen and analyze with tools like Imatest (or DXO's software). The moment you bring the sensor into play, there are significant enough losses of data, and you can only, at best, guess at what those losses are (unless you have some detailed inside knowledge about whatever sensor it is your testing with). Your article is an interesting start, but you are assuming a Gaussian PSF. An actual PSF is most definitely not Gaussian, nor is it constant across the area of the lens (i.e. it changes as you leave the center and approach the corners...do a search for "spot diagram" to see actual lens PSF's produced mathematically from detailed and accurate lens specifications...even for the best of lenses, outside of the most centeral on-axis results, a PSF can be wildly complicated). Not to mention the fact that you have to guess the kernel in the first place, so whatever your result, it is immediately affected by what you think the lens is capable of in the first place.
Personally, I wouldn't trust any site that provided "lens resolution" results reverse engineered from an image produced by any sensor. I would actually rather take the "camera system" tests than have someone telling me what their best guess is for lens performance.
QuoteOne 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!)
Of course those are lens+camera tests, and DXO never said otherwise.
Hmm, DXO's own description on the lens tests page begs to differ:
DxOMark's comprehensive camera lens test result database allows you to browse and select lenses for comparison based on its characteristics, brand, type, focal range, aperture and price.
Nowhere in there do they state that the camera sensor is a factor in your ability to select and compare lenses. They only state that the lens characteristics, brand, type, focal range, aperture, and price are the applicable factors.