For the strength of electromagnetic waves we discuss here we are in the region of linear optics. Linear means if i double the input the output doubles as well. This means the transfer curve of a lens is a perfect straight line. The glass in the lens does not saturate and limit the light nor does it add light.
Flare makes this complicated. A gray card will have a certain tone in a negative. This tone will change when flare comes into play. But the change comes not from the object itself (the gray card), it comes from another light source on a different position. The transfer curve becomes more complex and the output for every single point on the negative (i.e. its gray value) depends on all points in the object plane. Nevertheless this is still a linear function.