Adding guide numbers

If I'm using two different flash guns, both facing forward, how do I add the guide numbers together?
I have a feeling it will be the square root of the sum of the squares, but can anyone confirm or destroy that theory?

apersson850 wrote in post #6962170
I have a feeling it will be the square root of the sum of the squares,

Yep.

Or you could use the guide number of one and add one stop.

But that assumes they are both equally powerful, as far as I can understand. I don't have two identical flash guns.

The reason I'm asking is that I'll need to take photos of runners outdoors at night in a while, so I'll need all light I can get. Hence I thought about combining two units, one on top of the camera and one connected to the PC sync connector.

After that, the runners will probably not see where to run any longer...

Let's do this empirically...If you have two GN100 flash units, the result is one f/stop brighter than a single GN100 flash unit. So instead of 25' f/4, you have 25' f/5.6 and so the new GN is simply 25*5.6=140 This confirms your hypothesis about ' square root of the sum of the squares'

While I admit the math is great to know I would use my flash meter and measure it....

Knowing the math gives you the advantage of really understanding what's going on. A meter lets you set up and get on with it.

Titus213 wrote in post #6963173
While I admit the math is great to know I would use my flash meter and measure it....

Knowing the math gives you the advantage of really understanding what's going on. A meter lets you set up and get on with it.

I was thinking the same thing while reading the above posts. Doing the math is fun but metering the light is more productive - time wise.

I used to be able to calculate (in my head) fill flash outdoors, shooting with a Norman 200B - no auto anything. Now that was fun. I can still do it but I'm way too slow (old) for that now.

Unfortunately, I'm not the owner of any flash meter.

Besides, it wasn't that I couldn't expose properly. It works fairly well, actually. Just compensate the exposure a bit and I can take photos outdoors with reasonable result, in full auto.

The reason for the math was that I wanted to understand how much I really would get. One of my flashes has a guide number of 45 meters, the other 58 (when fully zoomed). Combining them and using ISO 400 instead, I get nearly 150 meters. That gives me a fair idea about the range I can achieve.

Understanding is definitely in order. It will even make the use of a light meter easier.

apersson850 wrote in post #6966010
Unfortunately, I'm not the owner of any flash meter.

Besides, it wasn't that I couldn't expose properly. It works fairly well, actually. Just compensate the exposure a bit and I can take photos outdoors with reasonable result, in full auto.

The reason for the math was that I wanted to understand how much I really would get. One of my flashes has a guide number of 45 meters, the other 58 (when fully zoomed). Combining them and using ISO 400 instead, I get nearly 150 meters. That gives me a fair idea about the range I can achieve.

While being able to compute precise combined GN is a useful skill, at the same time it is a bit of a waste of time! The reason I make that statement is simply that it takes double the power to increase light by one f/stop. And it takes four times the power to get a two EV improvement. Another reason why I made that statement is the GN is a fairly optimistic approximation that assumes ceilings and walls rather than open air, so that open air results end up typically underexposed.

If you take any second unit which is less that 1:1 ratio in guide number to the first light, you get less than one EV increase in output. So if you had a GN58 unit along with a GN43 unit, you know inherently that you are increasing power at f/4 to less than f/5.6 So I immediately know that I can go less than 1.4x as far, so if I could reach 14 meters before I know I get to approximately 18 meters (14 * 1.3). If we did the long math, we get 18.2 meters.

apersson850 wrote in post #6962313
But that assumes they are both equally powerful, as far as I can understand. I don't have two identical flash guns.

The reason I'm asking is that I'll need to take photos of runners outdoors at night in a while, so I'll need all light I can get. Hence I thought about combining two units, one on top of the camera and one connected to the PC sync connector.

After that, the runners will probably not see where to run any longer...

Oh. You're right. I was assuming identical units. In the words of Emily Litella, never mind.

Years ago I used two tandem mounted flash units with some regularity, but they were both Vivitar 283's. I actually used their external light sensors set the same and it worked.

apersson850 wrote in post #6966010
Unfortunately, I'm not the owner of any flash meter.

Besides, it wasn't that I couldn't expose properly. It works fairly well, actually. Just compensate the exposure a bit and I can take photos outdoors with reasonable result, in full auto.

The reason for the math was that I wanted to understand how much I really would get. One of my flashes has a guide number of 45 meters, the other 58 (when fully zoomed). Combining them and using ISO 400 instead, I get nearly 150 meters. That gives me a fair idea about the range I can achieve.

Thats good thinking.