Anyone know what spectrum cameras use to meter?

[sk66]

I have similar lenses from different manufacturers and sometimes the camera will give a different exposure for the same scene. One coming out notably darker. It's not a lack of contrast in one lens, neither image is "washed out"...the one will just seem somewhat underexposed.
I know of the general tendency of my camera (nikon D300) to overexpose and I've accounted for that. I understand more elements in a lens can/will reduce contrast, but that's not it. A lack of contrast results in either a washed out image, or lack of image detail and that's not evident.
The only thing I can see is different lens coatings affecting the light spectrum used by the camera to meter a scene. But if my camera has an RGB histogram, I assumed it was using RGB spectrum to meter....maybe that's not the case.