Question Question about cumulative GN for all strobes combined

[Hoag]

My 2 main strobes are a pair of Inon S2000s. They are great little strobes, but are each GN20, so reasonably adequate, but not terribly powerful. I have them mounted on arms to spread them out & reduce back scatter. I also have a Sea&Sea YS-03 (also with a power of GN20) that I have as a spare.

I realize that strobe placement with affect both the amount of back scatter and the "shape or quality" of the light. I also realize that even with an increase on the amount of light, I will still need to be no more than about a meter (3ft) typically for the strobes to really be effective.

Here is my question: If I add the YS-03 to my system (probably mounted on an arm off of the cold shoe mount on my camera where the focus light is often mounted), I will have 3 strobes each at a Guide Number of 20 for a total of 60. Would this give me a similar amount of light as a pair of strobes each rated at GN30?

To put it another way, to find out how much light I am getting from my strobes, will adding the Guide Numbers up to get a total give me a ballpark idea?

I am looking to increase the amount of light I am getting from my strobe system and I am trying to decide if the economical (cheep, frugal, cost-effective) way of adding a 3rd strobe (which I already have) will be a suitable solution, or if I am better to sell what I have and replace them all with a pair of more powerful strobes. (I am thinking either the Inon Z330 II or the Sea&Sea YS-D3 II as possible replacements.)

As a "follow on question: Does anyone use 3 strobes and if so, how do you like the results?

 

[Searcaigh]

I'm not sure how all the GN adds up but not being a light scientist I don't think all the GNs add up to 60 for 3 x 20.

I recently had a faulty YS-D2J replaced with a YS-D3 II and have been running a BackScatter MiniStrobe off the optical output with a snoot on it.

Only used one 6" arm so far, but thinking of adding another for more flexibility.

 

[Hoag]

I'm not sure how all the GN adds up but not being a light scientist I don't think all the GNs add up to 60 for 3 x 20.

Thanks. I figured that I would be hoping for too much to have it a simple relationship such as this.

How do you like your 3 light set up and also how do you like the YS-D3II?

Another factor that I didn't even address in my original post is size. It would be nice to be able to continue getting everything into a carry-on. Two more powerful strobes may be easier to do this than 3 less powerful ones even if the less powerful strobes are physically smaller.

 

[Searcaigh]

How do you like your 3 light set up and also how do you like the YS-D3II?

The YS-D3II is good so far, it's only had 24 dives since I got it mid November, power output seems on par with the D2.

I've only used the three strobe set up three times so far, and if I dive on Sunday morning I'll be snooting again with an additional arm.

Another factor that I didn't even address in my original post is size. It would be nice to be able to continue getting everything into a carry-on. Two more powerful strobes may be easier to do this than 3 less powerful ones even if the less powerful strobes are physically smaller.

I had my SLR rig and a TG6 Rig with two YS-D2Js and two MiniStrobes in my carryon when I was in Indonesia last April and last October. The YS-D3 is about the same bulk as the D2

 

[tursiops]

The combined guide number is the square root of the sum of the squares of the guide numbers for the different strobes. So 3x GN20 means GN35. 2x GN30 means GN42.

 

[Hoag]

The combined guide number is the square root of the sum of the squares of the guide numbers for the different strobes. So 3x GN20 means GN35. 2x GN30 means GN42.

Thank you! That answers the big question. I had no idea of what the math involved would be. The math looks very simple, but if you don't know the equations to use (which I didn't), you don't have a starting point for the math.

Now, it is a matter of deciding which offers the best compromise, less total light, but perhaps more flexibility in how that light is "shaped" (with 3 strobes) or more light and essentially the same flexibility that I currently have (dual strobes on their own arms - a "standard" set-up). Both have their pros and cons.

 

[Searcaigh]

Personally I'd get two higher GN strobes rather than use three for normal shooting.

 

[Hoag]

Personally I'd get two higher GN strobes rather than use three for normal shooting.

That is the way I am leaning.

 

[arew+4]

That is the way I am leaning.

I think the third arm underwater would be a rather large PITA. The relatively small gain, 28 to 35 vs 28 to 42 weakens the case further.

 

[tursiops]

I think the third arm underwater would be a rather large PITA. The relatively small gain, 28 to 35 vs 28 to 42 weakens the case further.

Agreed. half a stop extra light for fussing around with a third arm is not a good tradeoff.