It may be worth pointing out that in multiple flash set-ups like that, then the flash power is not additive. In other words using 4 flashes of GN 30, say, is not the same as using 1 with a GN 120. It may only equate to GN 90 or so. There is a formula but I don`t know it exactly. Things are not always what they seem.
I`m not sure about this. Yes, one stop is twice the light, two stops is four times the light, but i don`t think it works that way. It`s kinda like the inverse square law but in reverse, if that makes sense, you need four times the power to get twice the light. Not that it matters. Anyone out there got a PhD?
2x gives you 1 extra stop. 4x gives you 2 extra stops.
If all the lights are pointing in different directions, this extra power won't automaticaly translate into extra stops at the SB face, but it could/should make the front face coverage more even.
Thanks BrianO, this has been bugging me. Using this formula means that using 4 flashes with GN30 is equivalent to using 1 flash of GN60 which is an increase of only 1 stop! Oops! That means, if my calculations are correct, that wedding and event photographers using light-sticks and the like with 2 speedlights are only getting an increase of 1/3 stop, although gaining a potential greater spread of light. Best not tell them !
gardenvalley wrote:
Thanks BrianO, this has been bugging me. Using this formula means that using 4 flashes with GN30 is equivalent to using 1 flash of GN60 which is an increase of only 1 stop! Oops! That means, if my calculations are correct, that wedding and event photographers using light-sticks and the like with 2 speedlights are only getting an increase of 1/3 stop, although gaining a potential greater spread of light. Best not tell them !
You must have something wrong...flash power simply put gives you one f-stop every time you double the power. You can prove it for yourself--set your flash to 1/4 power and find an aperture that gives you the proper exposure. Now set it 1/2 power. You'll be overexposed by exactly 1 stop. Close your aperture one stop and you're at the correct exposure again. You can do this all day long across the power range. If you have one flash at full power that gives you f8, and double the light on the subject with another equal flash at full power, you've just doubled your power and would get f11...
This might be of interest: http://photography-on-the.net/forum/archive/index.php/t-271010.html
Scott Clark wrote:
You must have something wrong...flash power simply put gives you one f-stop every time you double the power. You can prove it for yourself--set your flash to 1/4 power and find an aperture that gives you the proper exposure. Now set it 1/2 power. You'll be overexposed by exactly 1 stop. Close your aperture one stop and you're at the correct exposure again. You can do this all day long across the power range. If you have one flash at full power that gives you f8, and double the light on the subject with another equal flash at full power, you've just doubled your power and would get f11...
This might be of interest: http://photography-on-the.net/forum/archive/index.php/t-271010.html...Show more →
I`m getting a sore head. I agree,Scott, about doubling the power of 1 flashgun is increasing the exposure by 1 stop, but this refers to more than 1 light source and the power output of a flashgun is not rated in stops. I read the link you posted, but I think they may be wrong. I refer back to BrianO`s post. If this is true then a lot of people are not going to like the numbers. You can`t change the laws of physics, Captain! Does anyone have 2 identical flashes and a light meter? Now where did I put that box of Paracetamol.............
gardenvalley wrote:
I`m getting a sore head. I agree,Scott, about doubling the power of 1 flashgun is increasing the exposure by 1 stop, but this refers to more than 1 light source and the power output of a flashgun is not rated in stops. I read the link you posted, but I think they may be wrong. I refer back to BrianO`s post. If this is true then a lot of people are not going to like the numbers. You can`t change the laws of physics, Captain! Does anyone have 2 identical flashes and a light meter? Now where did I put that box of Paracetamol................Show more →
I've got a pile of flashes and a meter...at home . If nobody beats me to it, tonight I'll put the smack down with a couple of SB-28s and see exactly what happens .
Good on you, Scott. If the formula is right, make sure there`s something nice and soft to prevent your jaw from hitting the floor. I am now intrigued to see if the real world matches the numbers.
gardenvalley wrote:
Good on you, Scott. If the formula is right, make sure there`s something nice and soft to prevent your jaw from hitting the floor. I am now intrigued to see if the real world matches the numbers.
. Will do. I'll eat my hat if adding a second flash only yields 1/3 of a stop .
Thanks for coming in on this one Carmen Miranda. I again refer you to BrianO`s formula. If anything, this thread indicates the confusion which reigns regarding the science of light. Although not entirely scientific, let`s wait and see what Scott comes up with. We`re trying to ascertain how much light is gained by using 4 flashes rather than 1. What we have to bear in mind is that light propagation is not linear but the ratio we use to measure exposure is; 2.8, 4, 5.6 etc. I`m now going to go and lie down in a dark room with my teddy bear.
gardenvalley wrote:
Thanks for coming in on this one Carmen Miranda. I again refer you to BrianO`s formula.
In the interest of accuracy, "my" formula came from Light -- Science and Magic, 3rd Edition, and is found in chapter 10 on page 280.
Note that the formula is dealing with guide numbers, not f stops. So four strobes only doubles the range compared to one flash -- a demonstration of the inverse square law at work. To double the guide number requires a quadrupling of the flash output, because the area increases at the square of the distance.
The way to test this would be to measure the f stop required for a given exposure with one strobe at a given distance, and then measure the distance at which four strobes gives the same f stop.
I`ve figured it out. Given that the above formula is correct;
example: GN 40, distance to subject 10 feet, f-stop would be 4
GN 80, distance to subject 10 feet, f-stop would be 8
This is a difference of 2 stops, therefore using 4 flashguns instead of 1allows for an extra 2 stops. If I`ve interpolated this right, it means that Scott`s experiment using 2 flashes will show an extra 2/3 stop difference. It`s not that bad an increase but I think it will still surprise a few people. If nothing else I`ve learned something from this. Thank goodness for TTL and histograms.
. If nobody beats me to it, tonight I'll put the smack down with a couple of SB-28s and see exactly what happens 
. Will do. I'll eat my hat if adding a second flash only yields 1/3 of a stop 
.