Question for Voigtlander 35/1.2 owners...

Let's say you spot meter on a flat color surface, then slowly open up the aperture from 1.8 to 1.2. Does your camera's meter properly reflect the aperture changes for each step, or do you find that you are losing some light the wider open you go?

If there is some light loss, I am also wondering if the effect is the same, or different, between various bodies (actually, I'm more interested the behavior on the NEX-7)?

Basically, I am trying to evaluate this lens and trying to gauge just how much actual gain I will see.

Thanks in advance!