David Farber wrote,on my timestamp of 17/09/2008 9:01 AM:
I was wondering about how fill flash affects the existing light. Of course
it will take the shadows and lighten them up a bit, but what about the parts
of the picture that are correctly exposed by the existing light? I mean the
light emanating from the flash (attached to the hot shoe) does not just go
to the shadows. Wouldn't the areas not in shadow then be overexposed by a
half stop? My fill flash photographs (from print film) have been coming out
nicely but this conundrum has me scratching my head. On second thought,
should I be using some ratio method to compensate for the existing light
exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
3600HS flash unit.

What you forgot is why/how folks use fill flash.
Yes, it's additive. Have a look at most examples
in manuals for fill-flash: it's usually a portrait of a
person in shadow, with a light or shadow background,
AWAY from the main subject. That means if you shine the
flash at the subject, it'll light it up ok but the
background will be too far away to be seriously influenced
by the amount of flash light used.
Flash lighting is very dependent on distance from the flash,
given constant intensity.