I have the variable model and a while ago I did a test just for the heck of it. I measured the light intensity at the middle and edges under various setups and the 35mm (50 lens) did not change AT ALL in evenness except that it was better than medium format sizes (80&105 lenses) and much better than larger format (135) for even light distribution when wide open or one stop down. It was in fact even edge to edge just one stop or more down, whereas all the other combos had to have three stops down before the edges caught up with the center.
The price for that however were longer times. If I remember right (wouldn't count on it) the times were 1.7 times longer.
The problem is when you use a small condenser set with a larger negative. Horrible falloff.