Ok, I was able to figure this out. My experimenting is telling me that focal length really has nothing to do with distance falloff, but it is the scale of the camera that is the true issue. As petercat pointed out in the last thread, when you adjust the focal length, Poser automatically alters the scale and distance of your camera. Well, this is true for the Main, Aux and Posing cameras (and probably several of the others), but this doesn't happen on the Dolly Camera. On the Dolly Camera, you can adjust the focal length without effecting any of the other dials. Also, on the Main Camera (as well as the others that work like it...Aux and Posing) you can change the scale independantly of the focal length.

Lets say you're using your main camera. Focal at 35mm, Dolly Z at 10.000 (keep X and Y at 0 to keep it simple) and Scale at 100%. Now change your Focal to 70mm and Poser automatically adjusts Dolly Z to 5.000 and Scale to 200%. If you were to render now, your falloff settings would not give you the desired results. But, now we can change the Dolly Z back to 10.000 and the Scale back to 100% and the Focal length will stay at 70mm. Now render, and you will see your desired falloff results.

Of course if you use the Dolly camera, adjusting your focal length never messes with your scale and you don't have to worry about this at all. So my question now is, why does it have to mess with your scale and distance on the Main, Aux and Posing Cameras?

(P.S. bloodsong, I've seen your work. I think you know more about lights and cameras than you think you know.)