Multiple fashes and guide numbers

Canon EF 17-40mm f/4.0L USM

12/15/2005 07:26:14 PM · #1

While I'm thinking of flashes, if I have, say, two flashes with GNs of 20m @ ISO 100 and I fire them in sync would I have an effective GN of 40m @ ISO 100? Or doesn't it work like that...

soup

Canon EOS-30D

12/15/2005 07:44:00 PM · #2

../

i dunno what i am talking about ;}

Message edited by author 2005-12-15 19:45:15.

Canon EF 70-300mm f/4.5-5.6 DO IS USM

12/15/2005 07:49:54 PM · #3

Nor do I, thats the problem :) I just realised that my first thoughts can't be right because although I am doubling the initial output there is a inverse square law going on too, and then my head just begins to hurt and I give up...

cpanaioti

Canon EOS-40D

12/15/2005 07:50:41 PM · #4

I don't think two of them would increase the reach. Logically I think it would just increase the amount of light within original 20m.

I have no references for you so can't confirm/dispute this theory.

If you're using them at different angles/locations, what does that do to the equation?

12/15/2005 07:54:31 PM · #5

Originally posted by cpanaioti:

If you're using them at different angles/locations, what does that do to the equation?

Mainly hurts my head more :)

fotomann_forever

12/15/2005 07:59:57 PM · #6

I can tell you from experience that No, it won't double your reach, I've tried it shooting night races at a local track. It WILL increase reach some, but it's not nearly as much as you'd think.

In that situation... to increase my "virtual" flash range, I slaved flashes closer to the track (put the flashes in harms way instead of my body) ...

I dunno, why you want to increase range that much, but slaving might help, just a thought.

12/15/2005 08:03:54 PM · #7

I don't really want to increase the range, I just needed to work out what the GN would be when using two flashes instead of one so i could set everything correctly.

Canon EF 70-300mm f/4.5-5.6 DO IS USM

12/15/2005 08:50:39 PM · #8

OK, after a bit of googling I have an answer,

"For multiple flash, the GN of the combined guide number is the square root of the sum of the squares of the GN of each individual flash" from here

So, in the case stated in the original post,
20^2 + 20^2 = 800 and the square root of 800 is ~28.3 so my new GN would be about 28m (84ft) @ ISO 100

Prof_Fate

12/15/2005 08:55:50 PM · #9

Originally posted by james_so:

the internet is so cool!

cpanaioti

Canon EOS-40D

12/15/2005 08:57:05 PM · #10

Originally posted by james_so:

Is this with the two flashes in the same location?

Canon EF 17-40mm f/4.0L USM

12/15/2005 09:01:22 PM · #11

Originally posted by cpanaioti:

Originally posted by james_so:
OK, after a bit of googling I have an answer,

"For multiple flash, the GN of the combined guide number is the square root of the sum of the squares of the GN of each individual flash" from here

So, in the case stated in the original post,
20^2 + 20^2 = 800 and the square root of 800 is ~28.3 so my new GN would be about 28m (84ft) @ ISO 100

Is this with the two flashes in the same location?

Yes, or at least the same distance from the subject, which for my purposes will do the trick nicely..

soup

Canon EOS-30D

12/15/2005 10:09:03 PM · #12

so i did know what i was talking about... ;}

downtherabbithole

Canon EOS-30D

Canon EF 28-135mm f/3.5-5.6 IS USM

12/15/2005 11:49:31 PM · #13

it's all a game of math. you also have to know your stop factor charts. you can gain that but you will have to pop the flashes more than one time in the exposure. if you want all the math pm me, I can teach you if you want to learn that bad.