Combining flashes

selmerdave · Jan 17, 2006

I'm wondering about how to calculate guide numbers when using more than one flash. I did some artwork copying last week and used two flashes set for GN19, about two meters from the artwork and about a meter-and-a-half from each other with the camera in between. I thought the guide numbers would combine (for a total of GN38 ), but my results clearly show that is not the case. Luckily I bracketed and shooting at ~f13 (GN26) gave fairly good results, but still a bit underexposed in some cases. What is the correct way to calclulate it? Thanks.

Dave

Digital Matt · Jan 17, 2006

I would just use a handheld flash meter, for the best results.

Hertz van Rental · Jan 17, 2006

f-number for two flashes = 1.4 x (Guide Number/Distance)
where the distance of flash to subject, and the Guide Number, is the same for both flashes.

For 4 flashes:
f-number = 2.0 x (GN/Distance)

selmerdave · Jan 17, 2006

Thanks Hertz. I guess my number of GN26 is pretty close.

Another question: To ensure adequate coverage, relatively soft light and also to get the desired output, both flashes were set relatively wide. They weren't identical which no doubt is less than desireable, but they matched output with one set to 24mm coverage and the other set to 35mm coverage. I was shooting for the most part with a 105mm lens, and the subjects were generally somewhere in the 12" x 14" size range. I adjusted the flashes up and down and kept them firing at the center of the subject, but in several cases (but not all ?) there was fall-off at the edges. I kind of thought having the flashes so wide was overkill, but aparently not. Any thoughts? Thanks.

Dave