Quote Originally Posted by Bruce Osgood View Post
One additional area of concern is Filter Factoring, how much additional exposure to use. It is my understanding that when stacking filters you do not combine the factors, a 1.5X and a 2X do not make a 3.5X. Use only the single largest Factor for the stack. I don't think the order is important.
It should depend on how much the curves overlap, I think. If you had two filters with the exact same passband but different filter factors, you'd invert the filter factors, multiply, and invert again, so the 1.5x and 2x would come to a 3x. Of course if they didn't overlap at all, the filter factor would be infinity because no light would pass; and somewhere in between are most of the realistic situations.

To get a real answer for a particular pair of filters you need to consider their full response curves, but I'm pretty sure that using only the highest filter factor for the stack will always cause underexposure to some degree. Do I miss something?