mike@asgb.UUCP (12/20/84)
> The question was asked about exposure for multiple flash. One unit > GN 66 along the lens axis, the other unit at 45 degrees off axis GN > 120 You can easily figure out the correct exposure by thinking in terms not of guide number, but of units of light output. If your small flash (gn 66) puts out, say, 1 unit of light. The big one (gn 120) puts out 4 units. (Remember doubling the GN quadruples the light output.) So your highlights will have 5 units of light on them, and your deepest shadows have 1 unit. This assumes that the room has no significant reflections, which isn't true, and will smooth out the lighting ratio slightly. As a starting point for exposure, I'd compute the base exposure from the main light at a guide number of 120, and then stop down only about 1/3 to 1/2 stop. As to the umbrella, I think that a simple GN change would be adequate, if not exactly correct. It's more difficult to guestimate how much of a change it requires. I would direct some kind of continuous light into the umbrella, much like the flash would be aimed. A photo flood would probably be best, but I'm sure there are reasonable substitutes. The same color temp as the flash is preferable, but not likely. Use an incident meter to compare light output reflected off the umbrella compared with direct light. That gives the number of f/stops correction. Without an incident meter, take a reflected reading off an 18% grey card, or even your hand. With the umbrella, the small flash directly at the subject would likely deliver more light. You'll want to either diffuse, or get rid of the smaller flash. Mike Rosenlof ...hplabs!sdcrdcf!\ -bmcg!asgb!mike ...allegra!sdcsvax!/ Burroughs Advanced Systems Group Boulder, Colorado