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Shouldn't a lens be good or bad independently of the camera it is used with?
After seeing these reviews at DxOMark I got confused. By changing the camera where the lenses are tested I get different scores. I'm not talking about DX vs FX, because for sure something must differ from a full-frame to a cropped frame. But a score changing from a D5000 to a D7000 to a D90... why?
What is the influence of the camera on the image quality and performance of a lens?
A lens always performs the same way on all cameras with the right mount. What changes is the output from the camera which is dependent on both the lens and the camera.
Think of the light coming through the lens and then passing into the camera. The lens is first in the chain and always does the same thing.
Then, the light reaches the sensor which samples the light. A higher resolution sensor will be more demanding on a lens. There is also often an anti-alias filter in front of the sensor itself. This blurs the incoming light to avoid moire problems but that also means a stronger anti-alias filter will reduce the performance of top-quality lenses. For this reason, medium format systems (and Leica's own) do not use such filters.
The five factors listed are all functions of the lens, as you rightly point out, but the resolution of the camera may also have some bearing in the perceived quality of the lens, especially if you start pixel-peeping. A higher resolution sensor will show greater chromatic aberration at 100% magnification (i.e. when the sensor pixels are mapped one-to-one onto a display) than one with a lower resolution, and if the scores are based on this then that would explain the difference. The same applies to the lens resolution - a lens would be sharper when used with a lower resolution sensor than with a high resolution sensor.
Remember that any test or review you see is a about that particular copy of the gear; performing a test or reviewing equipment based on a statistically valid selection would be insanely expensive. Often quality of the same lens varies slightly, which is you may get different measurements or conclusions for the same lens in different tests.
