calculating flash output question...

hey guys,

As i'm getting more and more into using flashes, my mind starts to wander :P Forgive me if its a dumb question.

Say I have one flash, at 1/4th power. Without factoring in the stops of light lost by using a modifier of some sort, lets say we add in another flash at 1/4th power also.

Will this give me a flash output of a single flash at 1/2 power? Or is there a different ratio for light?

I'm curious as to if I could somewhat overpower sunlight a bit more with a hotshoe flash without a studio strobe. (ie, fire 4-5 hotshoe flashes into one giant umbrella)

thanks

e r y k wrote in post #5052819
hey guys,

As i'm getting more and more into using flashes, my mind starts to wander :P Forgive me if its a dumb question.

Say I have one flash, at 1/4th power. Without factoring in the stops of light lost by using a modifier of some sort, lets say we add in another flash at 1/4th power also.

Will this give me a flash output of a single flash at 1/2 power? Or is there a different ratio for light?

I'm curious as to if I could somewhat overpower sunlight a bit more with a hotshoe flash without a studio strobe. (ie, fire 4-5 hotshoe flashes into one giant umbrella)

thanks

Yes.

e r y k wrote in post #5052819
Say I have one flash, at 1/4th power. Without factoring in the stops of light lost by using a modifier of some sort, lets say we add in another flash at 1/4th power also.
Will this give me a flash output of a single flash at 1/2 power? Or is there a different ratio for light?

Yes. Light is energy and simply adds up (assuming it's uncorrelated which light is most of the time unless you use very expensive special laser systems).

Main condition is, of course, that both light sources shine at the same spot ...

It's a very expensive but doable way of accomplishing your goal. If E-TTL is a must, it's the way to go.

PS: I just noticed that jcolman beat me

e r y k wrote in post #5052819
Say I have one flash, at 1/4th power. Without factoring in the stops of light lost by using a modifier of some sort, lets say we add in another flash at 1/4th power also. Will this give me a flash output of a single flash at 1/2 power?

That's correct. Two identical lights, each at 1/4 power, will equal the output of one light at 1/2 power (ignoring probable differences in modifier efficiency).

Keep in mind that you need to double the power to get one stop more light. So if one flash gives you f/4, two flashes will give you f/5.6 and it will require four flashes to get f/8 .

Also keep in mind that four Sunpak 383 flash units combined will cost $320, weigh 3.8 pounds with batteries, and provide roughly 160 watt-seconds of power (not to mention you'll need to make the mounting hardware and find a way to trigger them all).

A single Alienbees B800 costs $280, weighs 2.9 pounds and provides 320 watt-seconds.

e r y k wrote in post #5052819
hey guys,

As i'm getting more and more into using flashes, my mind starts to wander :P Forgive me if its a dumb question.

Say I have one flash, at 1/4th power. Without factoring in the stops of light lost by using a modifier of some sort, lets say we add in another flash at 1/4th power also.

Will this give me a flash output of a single flash at 1/2 power? Or is there a different ratio for light?

I'm curious as to if I could somewhat overpower sunlight a bit more with a hotshoe flash without a studio strobe. (ie, fire 4-5 hotshoe flashes into one giant umbrella)

thanks

Well, the answer would depend on how the lights are set up or arranged. If the lights overlap each other, then the parts where they overlap would have a light equivalent of 1/2 power but the parts that do not will still be at 1/4 power.

thanks for the replies guys

the only reason im asking is that i already have 3 sb-28's, a 580ex and 430ex.

although the definition of portability is borderline if i bring all the flashes out, i still do not need a vagabond or a generator to power these lights :P

maybe i will look into studio flashes in the future, but for now I will stick with hot shoes.

one last question, how many watt seconds is the sb28 rated to? i cant seem to find anything online.

thanks for the quick responses looks like im going to have to rig up a mount for these flashes to fit under one umbrella. then pray my 8ft light stand will hold them!

e r y k wrote in post #5053154
thanks for the replies guys

the only reason im asking is that i already have 3 sb-28's, a 580ex and 430ex.

although the definition of portability is borderline if i bring all the flashes out, i still do not need a vagabond or a generator to power these lights :P

maybe i will look into studio flashes in the future, but for now I will stick with hot shoes.

one last question, how many watt seconds is the sb28 rated to? i cant seem to find anything online.

thanks for the quick responses looks like im going to have to rig up a mount for these flashes to fit under one umbrella. then pray my 8ft light stand will hold them!

I'm in the same situation as you. I have a 550EX, two sunpac 383's, stands, umbrellas and three pocket wizards for location lighting. I like to keep it quick and simple (i.e. light weight and no power cords) However I'm looking for a softbox that I can fire the two 383's into.

e r y k wrote in post #5053154
one last question, how many watt seconds is the sb28 rated to? i cant seem to find anything online.

Can't tell you the exact number, but I read something about GN to Ws conversion a while back. If your flash is somewhere in the GN 40-60 range (@105mm focal length), expect between 30 and 65 Ws. Pretty weak compared to studio strobes ...

Rudeofus wrote in post #5058919
If your flash is somewhere in the GN 40-60 range (@105mm focal length), expect between 30 and 65 Ws. Pretty weak compared to studio strobes ...

There's no "conversion" between watt-seconds and guide number, for a couple reasons. One of the reasons is that the guide number depends on the coverage angle and the efficiency of the reflectors that give you that coverage.

However, when measuring the output from a silver bounce umbrella and comparing that to a studio light, I estimate the 580EX II has about 40 watt-seconds. The SB-28 is about a stop less powerful when you compare guide numbers at similar zoom settings so you could guess at 20 watt-seconds.

Just keep in mind that's a very rough approximation. But my estimate seems to jibe with the numbers Rudeofus posted, so it's probably in the ballpark.

Curtis N wrote in post #5059150
There's no "conversion" between watt-seconds and guide number, for a couple reasons. One of the reasons is that the guide number depends on the coverage angle and the efficiency of the reflectors that give you that coverage.

You may not be easily able to convert Ws into a GN or GN into Ws, but it should be possible to estimate a Ws number for any given flash. The only question which remains is whether you are able to couple the whole energy into your diffuser but that's up to the person setting it up.