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Archive 2012 · Fill lighting question
  
 
IanWorthington
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p.1 #1 · Fill lighting question


This is really a photographic science question I guess, but is something I've been thinking about for a while and can't find an explanation...

When I fill light a subject I'm adding light to the shadows. But I'm *also* adding the same amount of light to the highlights. So why doesn't that cancel out the effect on the shadows?

Thoughts?

i



Aug 14, 2012 at 06:00 PM
cwebster
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p.1 #2 · Fill lighting question


I suggest you buy and read the book "Light - Science & Magic" it will explain how light works, how the inverse square law affects brightness, key : fill ratios and many other things.

As far as your question goes, if you add enough fill that the key:fill ratio reaches 1:1, you will have washed out the shadows and they will disappear. Remember that as you add brightness to the fill (and presumably to the key) you are increasing the overall brightness of the scene, requiring an exposure adjustment.

<Chas>



Aug 14, 2012 at 07:18 PM
RustyBug
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p.1 #3 · Fill lighting question


+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.

11:1

Now, add 100 photons to both sides and the numbers change to 1,200 and 200.
6:1

Add 900 photons to the original and you wind up with 2,000 and 1,000.
2:1

Add 1,900 photons to the original and you wind up with 3,000 and 2,000
3:2

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
1.001:1
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.


HTH

Edited on Aug 14, 2012 at 08:33 PM · View previous versions



Aug 14, 2012 at 08:16 PM
PeterP1D
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p.1 #4 · Fill lighting question


well explained


Aug 14, 2012 at 08:17 PM
 

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BrianO
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p.1 #5 · Fill lighting question


IanWorthington wrote:
...When I fill light a subject I'm adding light to the shadows. But I'm *also* adding the same amount of light to the highlights. So why doesn't that cancel out the effect on the shadows?


Chas mentioned the Inverse Square Law, and Rusty did a very good job of explaining how key:fill ratios come into play, but I'll my two cents' worth as well.

If we think about the Zone System, where we divide the dynamic range from black to white into 11 zones -- 0 through X with with Zone V being middle gray -- it would seem as though adding equal amounts of light to any Zone would increase the Zone value equally. In reality that is not the case, because the amount of light needed to increase exposure from one Zone to the next is not additive, but exponential. It takes more light to go from Zone IX to Zone X than it does to go from Zone 0 to Zone I.

Visualize adding candles to a room to make it brighter. Start with one candle. To double the light you'd go to two candles. To double the light again you'd have to go to four candles, not three. To double it again you'd need eight candles, then 16, then 32, then 64 and so on.

If you add one candle to a dark room you'd see a lot of difference, but if you add one candle to a 64-candle room you might not be able to see any difference.

I hope that makes sense, and I hope it helped.




Aug 15, 2012 at 01:00 AM
RustyBug
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p.1 #6 · Fill lighting question


+1 @ relative & exponential (vs. my linear analogy) ... good point


Aug 15, 2012 at 03:03 AM
IanWorthington
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p.1 #7 · Fill lighting question


Thanks guys. Let me have a think...

i



Aug 22, 2012 at 11:39 PM
wilt
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p.1 #8 · Fill lighting question


Let us start with the assumptions that

  1. EV1 is 'one part of light',
  2. EV2 is 'two parts of light',
  3. EV3 is 'four parts of light',
  4. EV4 is 'eight parts of light',
  5. EV5 is 'sixteen parts of light', a geometric progression.


'Four parts of light' is one EV brighter than 'two parts of light'. After all, EV3 is 1EV brighter light than EV2 !

Light is additive, like water...it accumulates. But the brightness accumulates according to the previously stated geometric progression.

If I start with 'one part of light' at both the fill side of the subject and on the highlight side, and then add 'two parts of light at the highlight side of the subject, the highlight side gets 1 part +2 parts, or a total of 3 parts of light, so it is 1.5EV brighter than the fill-only side (which gets 1 part of light).

If instead I start with 'two parts of light' at both the fill side of the subject and on the highlight side, and then add 'four parts of light at the highlight side of the subject, the highlight side gets 2 parts + 4 parts, or a total of 6 parts of light, so it is still 1.5EV brighter than the fill-only side (which gets 2 parts or light).



Aug 23, 2012 at 05:26 AM





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