 Denise Ms Goulet |
main-and-fill lighting If I use equal-strengh lamps for my main-and-fill lighting setup, and if I want to do a 2:1 ratio, does it means that if I place my main lignt at 4 feet from my subject, I will place my fill light at 8 feet from my subject? thanks!
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Alan N. Marcus |
Hi Denise, A 2:1 ratio lighting configuration is achieved when the light from the main arrives at the subject at exactly the same straight as the fill. Assume you are using a two light set-up and both are exactly the same as to wattage and reflector/umbrella. Let’s make-believe they are both 500 watt fixtures. The main with its 500 watts is set high and off to the side. The fill with its 500 watts is mounted near the camera at lens height. Now measure the main to subject distance. Place the fill at exactly this subject to lamp distance. If you follow my instructions both lamps will contribute 500 watts at the subject plane. This will be a 2:1 ratio. Why is this 2:1? The face and head are round with projection (nose) and valleys (dimples). Due to the main being off to the side, the side of the face opposite the main is in shadow. The shadows we are talking about are those cast by the main. Now the job of the fill is to illuminate these shadows from the camera’s prospective. With this configuration, the frontal surfaces of the face will receive light from both the main and the fill. The light from both is accumulative. Both contribute 500 watts so added together; the frontal areas receive 1000 watts. However the shadow areas only receive light from the fill. This is true because the shadows on the face are cast by the main meaning the light from the main can’t reach into these areas. So we have 1000 watts on the frontal areas of the face and 500 watts in the shadow areas. That’s what we call a 1000 to 500 ratio. We generally write this as 1000:500 (note the colon). Saying 1000:500 is award. We can reduce this ratio mathematically just like we can reduce a fraction. We must find a common denominator. In this case the common denominator is 500. Divide both sides of the ration by 500 and we get 2:1. You need to know that a 2:1 ratio is flat lighting. That means the resulting portrait has poor contract (it is said to be flat). We can improve the portrait rendering if we achieve a 3:1 ratio. To do this we must reduce the fill brightness to ½ as compared to the main’s brightness. We can do this by changing the fill lamp out to a 250 watt lamp. Better yet we can just move the fill back increasing the fill to subject distance. Brilliance (power) decreases with distance. If both main and fill are equal in wattage, we multiply the main to subject distance by 1.4 and place the fill at this revised (lengthened) distance. Now the main continues to contribute 500 watts however the fill now contributes 250 watts. The light on the frontal part of the face is 500 from the main + 250 from the fill = 750 watts from both. The shadows receive only 250 from the fill. The ratio is 750:250 which reduces to 3:1. Comprehend with caution; I only give marginal technical advice.
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anonymous |
Just remember though. If you have both lights at 4 feet from your subject, and you move the fill back another 4 feet, then your light strength from the fill hasn't halved, it has gone down to a quarter. I can't remember what it is called (I should know considering it was in my half yearly exam).
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Alan N. Marcus |
Hi Natalie - Denise, It’s called the law of the inverse square. According to the law, each time we double the distance, lamp to subject, the light at the subject plane diminishes to ¼ of its original intensity. This is a massive change equal to 2 full f/stops. Better control is had by movements yielding a 1 f/stop change. This is accomplished by multiplying the lamp to subject distance by 1.4. The answer is the revised distance to place the lamp to achieve a 50% reduction (1 f/stop). Conversely to gain one f/stop of brightness, we multiply the original lamp to subject distance by 0.707. This magic factor calculates a closer lamp to subject distance that accomplishes a 100% brilliance increase (1 f/stop more light). The law of the inverse square performs precisely when the lamp we are using consists of a bare bulb. When we use an umbrella or other diffuser the light is caused to radiate out from the entire surface of the diffusing fabric. This enlarged illuminating surface is now technically known as a “broad” light source. “Broads” do not follow the law very closely, instead, the light energy falls off gradually when we change lamp to subject distance. Thus if precise exposure adjustments are preferred these are best achieved using a light meter and not distance adjusting math. This is one of the reasons why a “broad” is the preferred lighting for a group shot. Groups tend to posed in rows thus using a “broad” as the main solves the problem of exposure differences arising from varying lamp to subject distances. Also a “broad” earns its keep when photographing children. The subject, illumined by a “broad” can be allowed to roam as the lightning level will remain stable throughout a greater span. Remember I only give marginal technical advice. Alan Marcus
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anonymous |
Thanks Alan! I couldn't remember the name of it and you explained it soooooo much better!
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John H. Siskin |
Hi Denise, Something to keep in mind when using ratio lighting, if you use large light sources, like soft boxes, you will not see any ration in the face, even if you use the lights in the right place and at the right power. If you want to see the traditional look of a ratio set up, two sides of the face at different densities, you need to use just reflectors with your lights. You can make very interesting lighting set-ups with ratios! Thanks, John Siskin
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Alan N. Marcus |
A tip of the hat to Mr. John Siskin for his clear explanation. Keep it up! Alan Marcus
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John H. Siskin |
Thanks Mr. Marcus, very kind of you I am sure.
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Debby A. Tabb |
Gives a nice warm look, controled light to take away or add shadows. Hope it helps, Debby
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