Me again.
According to the vivitar website, the guide number on my 283 is 120. I've been playing with my SB-28 and its funky zoom head, and having to calculate out new guide numbers for each zoom setting. The book for the SB-28 says to use corrections of 0.71 and 0.5 for asa 50 and 25 film (respectively). Are those absolutes, or do I have to find a different set of corrections for the 283? The reason I ask is that I want to use Polaroid P/N film, which is rated at 50, but I'm told to push to 25 for better negs.

Let's see with a guide number of 120 @asa 100 what's the GN of asa 50 and 25?

With 100asa @ 10ft we'd get a fstop of f12.

If we use Nikon's corrections and cut the GN in half for asa 25 we'd get 60. At 10ft we'd get an fstop of 6

Since fstops double ever other stop 6 must be two stops from 12 and asa 25 is two stops from asa 100. It seems to work out right.

Guide Numbers work on the square root of 2. If you want to increase the GN by one stop, multiply by 1.41414. If you want to decrease by one stop, multiply by .70707 or .71

Aha! So that's how it works...
Thanks. I can't believe I've gone all this time and never figured out how to use a flash manually. Its going to get really interesting when I try to use both flashes together. (Do I just add the guide numbers?)
Rather than bugging you folks, is there a good text that explains all of this (something I can put on my Amazon wish list?)

I gotta think aobut adding two together. Obviously if they were both the same say two #5s you would get twice the light, once stop more, so the GN would go up by 1.4x But if you wanted another stop, you'd have to add 4 bulbs (doubling the output again).

things get squirrelly and my hair starts to hurt when we start mixing bulbs (or flashes of different size) something tells me its something painfull like adding the log of the the GN.

I don't know of any book out there, When I do multi flashes (from the same direction) if one has a guide number of 320 and another of 380 or 420 I usually ignore the extra, consider it a 320 and go on. A little extra light never hurts a negative.

I would suspect there was a book done in the heday of flash bulbs to calculate multi flashes but I'm not aware of it. I don't think "Synchroflash photography" has anything on mixing bulbs.
As for something new to be found on Amazon, well I'm working on it, it'll be out by Christmas, I just am not saying what year.

<font size=-1>[ This Message was edited by: Les on 2002-02-15 15:44 ]</font>

<font size=-1>[ This Message was edited by: Les on 2002-02-15 15:46 ]</font>

Quote:

On 2002-02-15 14:50, Les wrote: I gotta think aobut adding two together. Obviously if they were both the same say two #5s you would get twice the light, once stop more, so the GN would go up by 1.4x But if you wanted another stop, you'd have to add 4 bulbs (doubling the output again). things get squirrelly and my hair starts to hurt when we start mixing bulbs (or flashes of different size) something tells me its something painfull like adding the log of the the GN. I don't know of any book out there, When I do multi flashes (from the same direction) if one has a guide number of 320 and another of 380 or 420 I usually ignore the extra, consider it a 320 and go on. A little extra light never hurts a negative. I would suspect there was a book done in the heday of flash bulbs to calculate multi flashes but I'm not aware of it. I don't think "Synchroflash photography" has anything on mixing bulbs. As for something new to be found on Amazon, well I'm working on it, it'll be out by Christmas, I just am not saying what year. <font size=-1>[ This Message was edited by: Les on 2002-02-15 15:44 ]</font> <font size=-1>[ This Message was edited by: Les on 2002-02-15 15:46 ]</font>

Uh, Les, there is a way. But first, since this started as a question about Vivitar 283s, I don't think that the claimed ISO 100/feet GN of 120 is quite true. I have two, and by my Minolta flash meter their GNs @ full power are 90 and 100, respectively. Interestingly, with the VP-1 the two both have GNs of 80 (almost right) at half power. At lower power settings they continue to deliver the expected power.

Your question was, in essence:

How does one calculate the correct f-stop when using multiple flashes?

The trick is very simple if you know every light source's GN (by test for flash + umbrella combinations, by test or by mfr's lying claim for umbrella-less flashes). Set up the lights, measure light to subject distance for each, and use normal GN arithmetic to calculate correct aperture (f/#) for each light used by itself. Overall f/# is
just the square root of the sum of the squares of the individual f/#s. This is parallel to the way you add GNs (GN for a bank of flashes is the square root of the sum of the squares of the individual flashes' GNs).

For example, say you have 5 flashes and as set up they want, respectively, f/8, f/8, f/11, f/11, and f/16 (bad setup, but hey, this
is just an example). Then you want f/25 = 1/sqrt(626) = 1/sqrt(64 + 64 + 121 + 121 + 256). Simple, eh?

Note that adding a small flash (e.g., wants f/5.6) to a big one (e.g., wants f/22) buys very little (the two together want f/22.7, practically speaking no different from f/22). Also, once you've got around 4 flashes of the same size another one buys you very little.

Cheers,

Dan