Strings method reminds me of Chuck Gardner. In case anyone is interested:
http://super.nova.org/DPR/Ratios/
His string method:
"Lighting ratios are pure mathematics based on the inverse square law which states that the intensity of the light decreases or increases by the square of the distance.
In practical terms if you double the distance you get 1/4 the light, a 2 f/stop difference. f/numbers are also a mathematical formula based on the focal length and physical size of the aperture and as it so happens the same progression of f nubmers 16, 11, 8, 5.6, 4, 2.8, 2 when translated into distances for you lights will produce a 1 -stop differnce in intensity.
So you get a piece of string, tie it to the flash head, then tie knots in it at 4, 5.6, 8, 11, and 16 feet. Put the 8' knot on the subjects nose and shoot a bracketed series of exposures with the flash set to manual power (full or 1/2 power) and find the f/stop in the series that give the best highlight detail. A white terry towel is a good test target because it makes it easy to see when the detail is blown out.
OK, now you have your baseline exposure. Let's say for example f/11 @ 8ft. Now if you move the light in to the 5.6' knot you'll know without metering that the exposure must be changed by - 1 stop, or f/16. If you move the light out to 11 ft. you'll know the exposure will need to be +1 stop, or f/8.
When two identical manual flashes are used the strings can be used to set lighting ratio. equal distances will yeild a 2:1 reflected highlight:shadow; ratio when fill is placed near the camera highlights will appear 2x brighter than the shadows. For a 3:1 reflected ratio any of the following combinations of fill/key distances can be used:
16/11, 11/8, 8/5.6, 5.6/4, 4/2.8
As with a single light you'd do a baseline test at your most commonly used shooting distance with both lights set for the desired ratio, using that f/stop as the baseline for adjusting if lights are moved in/out.
With practice one can judge the distances without the string. For example a typical arm span is about five feet so if you've got one hand on the key light and the finger of the other near the subject's nose you can figure the key light is at about 5.6 feet. Take three paces back from the subject and your camera and on-camera fill is at 8ft. Perfect 3:1 ratio.
CG
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