If you use a light meter at the subject you can measure approximately how long the light needs to illuminate an area to give it normal exposure. That will at least give you a rough idea of how much light to use to record on film. Then you can experiment with film and evaluate the results to get an idea of how to proceed.
To get a reading on the meter you’ll need to measure the light fairly close to the source at a measured distance, say 1 meter from the light, and determine a basic exposure that will record on film.
Then for a light-to-subject distance D, the illumination difference (uncorrected for reciprocity) at the subject in f-stops based on a known 1-meter intensity is
Δf = 2*ln(D)/ln(2)
For example, at D = 2 meters,
Δf = 2*ln(2)/ln(2) = 2 stops less than the 1-meter intensity
At D = 5 meters,
Δf = 2*ln(5)/ln(2) = 4.6 stops less than the 1-meter intensity
Now you can determine the approximate light intensity at the subject.
Compensating for the considerable reciprocity loss can only be determined by experimentation. If you state your film and developer combination, possibly others who’ve done this with the same film and developer combination might offer advice with respect to the reciprocity compensation.
Other folks have had useful results painting with light using a hand-held sensor-auto flash like a Vivitar 283 using the “test” button to fire it. The photocell on the front of the flash (used within its range) stops the exposure when it receives the correct amount of light reflected from the subject. In this case no metering or calculation is needed.
Last edited by Ian C; 09-14-2011 at 12:30 PM. Click to view previous post history.