Found this response on photo.net from someone asking about using 3 strobes to light a subject. Probably the most technical explanation ever that actually makes sense. For those of you who know a lot more about lighting than I do, please post if the following response is valid or if the person is talking out of their rear.

Alan Marcus , Jun 29, 2009; 01:28 p.m.

"In photography the lighting ratio is the key resource we have to control contrast. Consider; our media is two dimensional (except for specialized cases), thus we must create the illusion of depth with our lighting otherwise the picture appear flat. Consider an aerial photo of a telephone pole. From above it’s not much to look at. Shoot with a morning or evening sun and the elongated shadows produce an impression of depth and elevation.

Portraiture is the study of the human head, a round figure scattered with projections and valleys. We light to give an illusion of depth. Additionally most believe we should preserve an illusion that just one light was used (comes from a study of the master working in oil on canvas), if multiple directional shadows are detected, the picture will look weird. So we light using a main light fixture set high to simulate midday sun. We position the main in such a way that the shadows that result enhance features. Long nose; light with short nose shadow. Short nose; light with long nose shadow. Oval face; light from the front, to make the face appear more rounded. Fat face; light from the side, keeping the opposing cheek in shadow to give an illusion of a thinner face. Positioning the main is an acquired skill.

Now for contrast control: Both film and digital lack the dynamic range of the human eye/brain. When taking a picture we see detail in the shadows that the camera is unable to record. Thus for fine portraiture, we are forced to diminish shadows, this translates to filling them with light. Otherwise the shadows record void of detail. You are advised to fill shadows from the camera’s prospective. This is accomplished by positioning the fill lamp somewhere along an imaginary line drawn between camera and subject. Its elevation is set to lens height. OK if fill strays off this line as needed to prevent the fill fixture from getting in the picture.

We start with a two lamp set-up

Consider a portrait 2:1 (Flat Lighting)

We place the main high and off to the side causing it to shine down on the subject. Let’s say the main delivers 1000 units (watts if you like) on the subject. The fill is placed near lens axis and adjusted to deliver light to arrive at the subject plane with equal light energy as the main. Thus the main and the fill each contribute 1000 units of light. Now the frontal part of the face receives light from both. Thus the total on the frontal areas of the face will be 2000 units. Now some areas of the face are in shadow. These are locations where the main could not reach, we are talking about dimples and valleys and nose shadow etc. Consider the circumstances; 2000 units on the frontal areas and 1000 units in shadow areas. Mathematically this can be stated as a ratio. The ration is written as 2000:1000. This is handled like a fraction that van be reduced by dividing both sides by same number; in this case 1000. The reduced ratio is written as 2:1. This 2:1 lighting ratio is flat nearly featureless.

Now consider a portrait set-up 3:1 ratio (bread and butter lighting). This is the one that wins contests and sells best. To achieve we reduce the fill energy at the subject plane to ½ power as compared to the main. We might do this by setting, a knob on the fill lamp or we can just move the fill fixture further away from the subject.
We can calculate the fill-to-subject distance (assumes both main and fill are identical). We measure main-to-subject distance and multiply this value by 1.4. The answer is a revised fill to subject distance. This added distance reduces the light energy playing on the subject by 50% (1 f/stop). The 1.4 factor is derived from a law in physics known as the inverse square law. The idea is to cause the main to deliver 1000 units and the fill 500 units. Now consider the frontal area of the face get light from both fixtures. The values are 1000 main + 500 fill. Thus the frontal areas receive 1000 + 500 or 1500 units. Shadows receive only the fill’s 500. Ratio is 150:50 reduces to 3:1. This is achieved if the fill is subordinate to the main by 1 f/stop. You can also place main and fill using a meter. The trick is again 1 f/stop difference, fill subordinate.

Consider 5:1 somewhat more zippy lighting.

We reduce the fill to 1/4 power by knob on the unit if available or by setting the fill even further back. If the fill is at the 2:1 distance (same distance as main) we multiply by fill-to-subject distance by 2. If the fill is positioned at the 3:1 distance, multiply fill-to-subject distant by 1.4. You are after a two stop difference with the fill subordinate to the main. How is this 5:1? This placement causes the fill to be 2 f/stops subordinate or 25% of the main’s energy. Now the frontal area receives 1000 from the main and 250 from the fill for a total of 1250 frontal and 250 in the shadows. The ratio is 1250:250 or 5:1. This is contrasty lighting.


Consider 9:1 somewhat theatrical, very zippy lighting.


If we reduce the fill to 1/8 power by knob or measurement 3 f/stops subordinate to the main. From the 2:1 position the multiplier is 2.8 from the 3:1 position the multiplier 2, from the 5:1 position the multiplier is 1.4. Main continues to delivers 1000 units the fill 125 units. Thus the frontal areas receive 1125 the shadows 125. The ratio is 1125:125 = 9:1 This is a 9:1 exceedingly contrastry lighting considered theatrical. Note 9:1 is the maximum ratio. Any more reduction and the shadows will be void of detail.

To review:
Main at 4 feet fill at 4 feet ratio is 2:1
Main at 4 feet fill at 5.6 feet ratio is 3:1
Main at 4 feet fill at 8 feet ratio is 5:1
Main at 4 feet fill at 11 feet ratio is 9:1



The third lamp can be used to illuminate the background. It is placed low behind the subject aimed up at the background. Its distance to background determines the apparent density of the background. It can be made to appear white of gray or black. Lamp-to background distance also obeys the 1.4 factor method or knob settings or metering.



It is no accident that the lamp-to-subject distances follow the f/number set which is 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22.
Note each value going right is its neighbor times 1.4.
Each value going left is its neighbor divided by 1.4
I call this kind of math gobbledygook"

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Olympus 40-150mm f/4-5.6 (80-300mm conversion)
Olympus 70-300mm f/4-5.6 (140-600mm conversion)

Don't have time to read the whole thing, the above statement is really not accurate. It comes from emulating rembrandt's window light. The proper position of the main is much lower, so there is light in the eyes.

Stan Thanks! I stopped reading @ the 2:1 I actually couldn't take it anymore.
Im sure its got some good points in there somewhere though.

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Care to correct what else is wrong with it then for those of us who don't know?

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Havent read it to debunk or comment on but as Stan pointed out the method he first writes about will cause dark eye sockets.

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Looks like the rest is on ratios.... the standard in portraiture is 3-1, using a standard main/fill setup. 5-1 is pushing the limit, you tend to lose detail in the shadows. Larger ratios are just split lighting. Less than 2-1 is flat, and pretty boring. The goal with a ratio is to either shape the face or create drama, or both. With a main/fill setup, a 3-1 has one stop difference. The main is hitting the mask of the face, the fill hits everything. So you have one unit of fill, two units of main. Since the fill also hits where the main does, it's 2+1, 3-1.

If you read the rest of the paragraph it mentions you adjust the light for the effect you want.

"We position the main in such a way that the shadows that result enhance features. Long nose; light with short nose shadow. Short nose; light with long nose shadow. Oval face; light from the front, to make the face appear more rounded. Fat face; light from the side, keeping the opposing cheek in shadow to give an illusion of a thinner face. Positioning the main is an acquired skill."

__________________
Always think positive, but expect the worst and nothing bad will ever happen.

http://hanratty.smugmug.com

Gear:
Olympus E-30
Olympus 14-42mm f/3.5-5.6 (28-84mm conversion)
Olympus 40-150mm f/4-5.6 (80-300mm conversion)
Olympus 70-300mm f/4-5.6 (140-600mm conversion)

I think it's easier to adjust the position of the face. Corrective posing is usually more effective on faces than corrective lighting. What he calls lighting from the side is short lighting, which is very effective in shaping the face. If I were to correct a nose issue, it would be with posing. :cool"

So what he's saying isn't actually incorrect, it's just another way of doing things? Not harder or easier, just different.

__________________
Always think positive, but expect the worst and nothing bad will ever happen.

http://hanratty.smugmug.com

Gear:
Olympus E-30
Olympus 14-42mm f/3.5-5.6 (28-84mm conversion)
Olympus 40-150mm f/4-5.6 (80-300mm conversion)
Olympus 70-300mm f/4-5.6 (140-600mm conversion)

He's saying a little with a lot. I didn't read it in depth- just quickly scanned. I didn't see anything that was specifically incorrect with respect to theory and ratios. I have to agree with Stan that a 5:1 ratio is pushing it. That's pretty darn dramatic. My two light ratios are usually a strong (because I like contrasty images) 3:1. By that I mean that it's close a 4:1, but probably not fully there. Without a flash meter I couldn't tell you what exactly it is. When I setup up I use Identical strobes and cut the power to get an initial ratio. Then I back the fill off or push it up to fine tune to the look I want.

One thing I did notice however, was that while what he describes is in fact a setup using three lights, it's not what I would consider a three light setup. Three lights to me would be three lights on the subject. ie. main, fill, and kicker. Or, main, fill, and hairlight. A third light on the background doesn't necesarily influence the light hitting the subject, (especially in his description) therefore it's just creating a background effect. The same kinds of differences could be made by switching from a black to a white background...as an extreme example.

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