Quote Originally Posted by BradS View Post
Yes, I understand the relation between film speed and GN but does a ND filter imply a guide number change when the flash is used for fill? I would not expect it to. The ND filter is not covering the flash....and the flash doesn't know anything about the ND filter....

Assuming a given camera to subject distance, the flash output needed for fill is the same regardless of the presence or absence of the ND filter - No?
Yes, it does give a guide number change. Using a 3 stop ND filter will quarter your GN. It cuts both ambient light AND flash light going into the lens. It affects both in the same manner, so you can slap a 3 stop filter on and open your lens up 3 stops, in the real world. If you needed a 1/1000 speed to get your ambient balance correct at your chosen aperture, you could put a 3 stop ND filter on and get the same balance at 1/125.

If you're running around doing fill flash with 3 stop ND on and 400 speed film, you'd have to calculate flash exposure, as well as ambient exposure, like you were shooting 100 speed film. So, yes, it does effect flash GN.