Ambar, why don't you think this way?
Originally Posted by Ambar
Privided that the subject you are going to shoot is evenly lit. There's no shadow nor highlight in this case. It's like you are going to shoot a wall that is lit by the sun in daytime using flashlight fill. In this case, you will get a picture which is about 1/2 stops overexposed. However, it can be still reasonablly tolerable even if you shoot with a slide film. That is because in this case there's no highlight in your picture. 1/2 stops of overexposure is safely within the film's latitude, which is usually around +2.3~-2.5 stops with the slide films. So, the overexposure only matters if there exists some highlight areas in the field.
Now you are going to shoot a more realistic subject that has both highlight and shodow areas with it next. However, everyting is still at the same distance for simplicity in this case. You determine the camera's exposure setting according to the ambient light. The meter reads around +0 stop on the main area you are going to shoot, the meter also reads +2.5 stops on the highlight area and -3 on the shadow area in this case. And you use fill flash with -1 stop setting. Do you think the highlight area will become +3.5 stops overexposed by the flashlight fill? - It is not likely, the highlight area remains almost unchanged at +2.5 stops overexposure.
The thing is, you just can't add the stops when culculating exposure. You need to add them by their percentage figures.
+3.0 stops -> 800%
+2.5 stops -> 560%
+2.0 stops -> 400%
+1.5 stops -> 280%
+1.0 stops -> 200%
+0.5 stops -> 140%
+0 stop -> 100%
-0.5 stops -> 70%
-1.0 stops -> 50%
-1.5 stops -> 35%
-2.0 stops -> 25%
-2.5 stops -> 17.5%
-3.0 stops -> 12.5%
So in the case of "highlight+2.5 / main subject+0 / shadow-3.0", the outcome will be...,
highlight : ambient light 560%(+2.5 stops) + flash fill 50%(-1.0 stop)
=> outcome 610% (less than +2.7stops; almost unchanged)
main area : ambient light 100%(+0 stop) + flash fill 50%(-1.0 stop)
=> outcome 150 % (about +0.5 stops)
shadow: ambient light 12.5%(-3 stops) + flash fill 50%(-1.0 stop)
=> 62.5% (about -2/3 stops)
You can see that the fill affects more on the shadow than on the highlight. This is why people say "Flash fill is for controlling the shadow (contrast of the picture)".
Sorry for my bad English,