jrista said:
I've been on a rampage against DXO lens tests for almost as long as they've been around, and I've been QUITE vocal about that here in these forums. DXO lens tests frequently indicate that Canon lenses are better than the competitions, DESPITE the undue bias they give to Nikon lenses thanks to the D800. Doesn't change the fact that DXO's lens tests are a joke, again thanks to that "weighted scoring" they do that vastly overweights factors that don't play a big roll in IQ, and vastly underweights factors that do play a big role. They also use the word "transmission" to refer to what is really "aperture", and therefor ALL of their lens tests are massively skewed by the transmission factor.
I have a D300s and a Nikon 200-400mm VRI, which is a nice lens by anyone but DXO's measurements. They tested the VRII model which is basically the same lens with improved VR and give it a score of 12!
http://www.dxomark.com/Lenses/Nikon/Nikon-AF-S-Nikkor-200-400mm-F4G-ED-VR-II-mounted-on-Nikon-D300s__614
Now, if you put it on a D7000, it jumps to 14. On a D3 its 17, and on a D700 its also 17. On a D4, it jumps to 21, on a D600, it jumps to 24, and on a D800, its 25!
This is why you don't compare lenses across different camera models, much less across different brands. Testing lenses on a camera body gives results that only apply to that body or one with the same sensor. In general, the test methods will give higher numbers with more MP. The lens itself did not change and is no better or worse just because its on a camera with more pixels.
The reason is simple, the MTF of a image is a product of the MTF of the lens, The Body, the monitor or printer its viewed on, and even your eyes. Raising the MTF of any of these things will improve the image as long as the others don't change. Of course, DXO does not USE MTF, just because the entire photography world uses it, they invented their own number, MPIX.
Now, if DXO wanted to compare lenses between Canon and Nikon, they'd test them all on the same Canon body. That would give you at lease some comparison, but it still would not be accurate, since manufacturers cameras recognize a lens model and may make subtle adjustments to exposure at the edges.
That's why most knowledgeable lens testers provide a warning note that says don't compare on different models or manufacturers, a lens test on a D300s is only good for a D300s, but may be similar for a body with the same sensor.