back to the topic:
here a test i made to understand how different ND settings come out in the rendering.
sorry for the poor quality, but i forgot to set the rendering time higher than 2 hours...
as far as i can see, (and as tom said

) the rougher the mat, the less ND setting affects.
and, the color at roughness 0 disappears when ND is set higher than 3. -> it turns to a perfect mirror.
at 2bsdf layer plastic mat ican´t see any effect...

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Dual Xeon 3.6, 8 GB RAM, NVidia QUADRO FX 4000
64Bit XP, Cinema 4D 10 64Bit Edition