A defensible test for noise must hold exposure variables constant. Testing a 24-120 zoom against a 50mm f/1.4 is a bad test. Even testing one 50/1.4 lens against another may not be a good choice, even though it’s commonly done.
The very best noise test will use the same lens for every camera. This is not always possible, but it is possible to compare Nikon and Canon DSLR performance this way; the Zeiss ZF 100mm f/2 Makro-Planar is an excellent choice, offering high contrast across the entire frame. An adapter makes this easy.
Different lenses have different spectral transmission (color balance) which can significantly impact the transmission of color, especially at the blue and red ends of the spectrum, as well as away from the optical axis (away from the center of the frame). The difference can be quite pronounced—witness the Zeiss ZF 18mm f/3.5 Distagon compared with the Nikon 14-24/2.8 zoom (covered in Zeiss ZF Lenses), both in color and field illumination.
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