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?
If the ND filter is on the lens, it will decrease the brightness of both the sunlight and the flash equally - at the film plane.

But if you have enough flash power to, for example, achieve appropriate fill at 1/500 at f/8 (assuming EI of 125 and a leaf shutter) then a 3 stop ND filter will allow for the same result from 1/500 at f/2.8.