I've been asking myself this exact question lately. I have a C1 that is, I'll agree,quite the beast, but indeed an excellent contraption. Using my Turner-Reich 12-21-28 I've noticed that the 12" length (f/6.8) is significantly dimmer than the 21" (f/11) and that the 28" (f/16) is the brightest of them all (though this probably because the 8x10 ground glass is right in the middle of the hot-spot). Just the same, though, the 12" seems about 2 stops dimmer than even the 12". I've been wanting to get my hands on a Wollensak Ex.WA 159mm lens and have wondered how bright it would be or even if I could focus down close enough, or if I'll have to make a recessed board for myself.