Elementary flash math

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The last few days I've been working out how to use 3 flashes as a single light source using a tri-flash bracket. In the process, I've been doing some basic math to determine the power levels on all 3 units to give me a combined equivalent of what I would get with one strobe. When going from 1 to 2 flash units, this is simple: when using 2 strobes, cut the power to 1/2 of what you would set on a single unit. With 3 you have to bring out some elementary fractions.

Take the case where you have 1 flash unit set to 1/2 power. To make 3 flashes add to the same flash power, you will need 1 flash set to 1/4 power and the other 2 set to 1/8 power: 1/4 + 2x1/8 = 1/4 + 1/4 = 2/4 = 1/2. Yes, to get the 3 flashes closer to one another in output level, you could also do 1/4-0.7 (of a step) + 2x(1/8+0.3), but that would make any sane person's head hurt, so we'll keep it simple.

Here is one example to see how this really works.

Settings: Nikon D300 & 24-120 f4, ISO200, 1/125 sec, apertures as listed below.

f/4: 1 x 1/8 power

f/4: 1 x 1/16 power + 2 x 1/32 power


[More samples at this link, since they use complex HTML not reproducible here.]

Those with a keen eye will notice that the 3 flash samples are slightly brighter than the 1 flash equivalents. This can probably be explained best by noting that the single flash was a Nikon SB-700, while the 2 additional flashes that made up the trio were SB-600 units. In other words, the flashes weren't perfectly matched. As it turns out, a simple -0.3 step adjustment to the SB-700 in the 3 flash configuration achieves a very close match in the lighting level.

f/4: 1 x 1/16-0.3 power + 2 x 1/32 power
 
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Interesting, Eduardo. The linked article you did really demonstrates the principle. I wonder if the small difference between the single and the triple flash shots is because the three flash set up presents a somewhat more diffused light.
 
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I did not understand why using three flashes if you are going to set them to 1/4 or 1/8 power?

You could simply meter them of course.

But to compute multiple flashes:

If equal guide numbers, then combined GN is

sqrt(number of flashes) x GN of one.

If unequal guide numbers, then combined GN is

sqrt( GN1² + GN2² + GN3² ... )

This assumes they are aimed to cover the same area of course.
 
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Interesting, Eduardo. The linked article you did really demonstrates the principle. I wonder if the small difference between the single and the triple flash shots is because the three flash set up presents a somewhat more diffused light.
Interesting thought. That is a possibility, though since I was shooting insider a larger version (but not much larger) of my DIY light box, some of the light diffused from the edges doesn't come into play... Still, I think you may make a good observation. I think it will be more significant factor when I am using a larger modifier. These shots used a 43" umbrella in shoot-through configuration, but the real reason I want to figure out this tri-flash power math is that I want to use it in a larger modifier, a 60" umbrella softbox where I fear that even a single SB-900 would be under-powered to fill up the entire softbox's surface. I'll be doing more experimentation with this in the days to come.
 
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I did not understand why using three flashes if you are going to set them to 1/4 or 1/8 power?
As I explained to Hawk, this is a trial balloon leading to a situation where I do think I will need more flashes to power behind a larger modifier. And, of course, obvious advantages of lower power are faster recycle times, faster strobe pulse, and more shooting time on one set of batteries (granted, 3 sets in this case).

You could simply meter them of course.

But to compute multiple flashes:

If equal guide numbers, then combined GN is

sqrt(number of flashes) x GN of one.

If unequal guide numbers, then combined GN is

sqrt( GN1² + GN2² + GN3² ... )

This assumes they are aimed to cover the same area of course.
Now that goes a little beyond elementary math, doesn't it? :redface: But I know I will have to start doing these sort of mind-benders soon. For this trial I used 1 SB-700 plus 2 SB-600s, so the GNs are somewhat mismatched. However, I have a 3rd SB-600 on the way, and that hopefully will align GNs a little closer, though I realize there are still minor differences even among same-model units.
 
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I did not understand why using three flashes if you are going to set them to 1/4 or 1/8 power?
Depending on your intended subject, that may actually be a very good reason to use multiple flashes. That way you get much faster recycle and extended battery life.
 
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Depending on your intended subject, that may actually be a very good reason to use multiple flashes. That way you get much faster recycle and extended battery life.

OK, but I would set them to equal levels for that try. :smile:
 
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OK, but I would set them to equal levels for that try. :smile:
Would you still do so even if you got over or under-exposure and didn't have the latitude to move the light closer or farther from the camera or to alter the aperture because you wanted to control a certain DOF? These are not corner cases. :smile:

Actually if you had 3 flashes set to the same power (say 1/4) and had to adjust one of them down or up by 1/3 (0.3) step to optimize the exposure, that would be like having one big light with 3x the power and being able to adjust it by 1/9 step. Another advantage, perhaps? Food for thought.
 
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OK, suit yourself. :smile: But if my lights were at 1/4 and 1/8 power level, I think I could react to some degree of over/under exposure.
 
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OK, suit yourself. :smile: But if my lights were at 1/4 and 1/8 power level, I think I could react to some degree of over/under exposure.
Did that just last night. I now have empirical experience that it works very well. BTW, I was right to be concerned about needing more than one strobe inside my 60" Photek softlighter. A single SB-900 needed 1/1 power at f/8, ISO200 and about 3-4 feet away from the subject. Not the way I want to run things, so I added two SB-600s to the mix and adjusted their power levels just so to get the lighting I wanted, with the SB-900 now at 1/2 or lower power. Very nice light. I should have sample images to show soon.
 
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You're welcome. Wayne is a fount of good, solid knowledge, and I'm brimming with enthusiasm to try new things, even if they don't seem to make much sense. :wink:
 

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