Originally posted by amoringello er, um, thats the whole point.
Dunno where you guys come up with this stuff, and I don't know what language I'm writing in, but I believe I have been quite specific with what I wrote.
Anyway, like you said, its all basic photography 101. No black magic. Just go out and try it. You'll see ND has no special affect on ambient v.s. flash.
ND affects overall exposure. PERIOD.
It cannot get more basic than that. Try it. If anyone can prove that ND grads affect the physics of ambient light more than that of light from flash, they get my K5.
You're of course right on a Physics level, both lights are impacted the same way, but one light (the ambient) cannot be modified by the photographer, while the other (the flash) can be adjusted.
So, in effect,
using a ND filter does have a special affect on ambient v.s. flash, not by cheating those good old optics mechanisms, but just by letting you crank the knob up on the flash a little more.
EDIT : and thank you areidjr for illustrating my words, I was too lazy to do it... You used a ND2 filter, no? The ambient seems about one stop underexposed in the second one.
But to be precise, in your case, you could have used 400 iso with the same result. ND filters are a solution at base iso.
Granted, this is a quite strange way of thought : to freeze an action, sometimes, it's better to "slow" it down!