JDM von Weinberg , Dec 20, 2012; 04:07 p.m.
Here's another excerpt from the DxOMark page linked to. It helped me to figure out what the graph above was claiming to show:
For example the Sigma 35mm f1.4 DG HSM A when mounted on a Canon EOS 5D Mark II scores 17.2 P-MPix, on the Canon EOS 5D it scores a near perfect 12. Samyang produce a lens with the same specification: the 35mm f1.4 AS UMC. It is around 30% cheaper so it is worth looking at. On the Canon EOS 5D it scores pretty well at just under 10 P-MPix a difference which many photographers would be prepared to live with. When you mount the Samyang lens on the newer Canon EOS 5D Mark II however, the score is 11.5 P-MPix, a loss of 45% of the camera’s resolution. Effectively the results from the Samyang will be only marginally better from the Canon EOS 5D Mark II than from the Canon EOS 5D.
I leave the validity of the metric to those better qualified to judge, but there is not universal acclaim for DxO in other assessments of cameras and lenses.