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+1 @ "Light - Science & Magic"
But if you are a numbers guys, there are a couple of ways to think that might help you understand why it doesn't cancel out the effect on the shadows, but rather alters it relatively.
Part of that is going to depend on the angle of coverage and the areas of overlap between the key and fill lighting. But assuming from the way you based you question, it sounds like you are adding fill to all areas equally (key & shadow), with a broad coverage fill. Assuming this is your proposed question:
Consider that you are putting 1,000 photons on the key side from your key light. Consider that you have 100 photons of ambient light for both your key & shadow side. In that regard you would have 1,100 photons on the key side and 100 photons on the shadow side.
Now, add 100 photons to both sides and the numbers change to 1,200 and 200.
Add 900 photons to the original and you wind up with 2,000 and 1,000.
Add 1,900 photons to the original and you wind up with 3,000 and 2,000
Add 9,900 photons to the original and you wind up with 11,000 and 10,000
1.1:1 (very nearly 1:1)
Add 999,900 to each side and it becomes 1,001,000 and 1,000,000
This creates an imperceptible difference, even though the 1,000 photon difference remains due to the mathematical aspect of adding equal amounts to both sides.
So, while it may be true that you are adding equal amounts to both sides and raising the overall totals, it's kinda like having two kids on a see-saw where one kid weighs 80 pounds and the other weighs 40. The forty pound differential represents TWICE as much and the difference is very obvious, with one side sticking way up in the air.
Add a couple of 360 pound sumo wrestlers and the two sides now become 440 and 400 ... the extra forty pounds is only a ten percent variance and the see-saw will be very close to balanced (if it doesn't break first).
The greater the "relative" difference, the easier it is to detect. Adding fill reduces the relative relationship and thus reduces the contrast between the key and shadow areas, making that original 1,000 photon difference less noticeable. It "mathematically" remains (and adds to the overall) a 1,000 photon difference, but the relative difference is now much less.
Edited on Aug 14, 2012 at 08:33 PM · View previous versions