This article was first published using the metric system, but after a few requests I am republishing this article using feet.
After reading the book Light Science & Magic and reading David Hobby's Lighting101 and Lighting102 courses at Strobist.com, I came to realize that I needed to put some effort into being able to get close to the right lighting setup from the start of a shoot, rather than my current method of randomly playing with the settings of the lights and the camera until I magically get a good picture - which of course I can never replicate.
The caveat here is that the Guide Numbers are just that - a guide. They are a place for you to get started not the law. Over time you will be able to get to know what settings you like for any given set of circumstances.
NOTE: The only calculations needed here are multiplication - nothing else I promise - as I can't do much else myself!
For people like myself though who do not have enough time in the day to enjoy getting to be intimate with our cameras the Guide Numbers and the calculating factor table will help us get into or closer to the zone much faster.
I currently own two 580EX II and one 430EX and I wanted to be able to calculate the Guide Number of each of these units (which are in the manuals) and also a combination of the units when used together as one unit. Below are the results from my efforts.
The top 5 tables represent the Guide Number for the combination of the flashes listed. The final table shows you the multiplication factor to change the Guide Number based on if you are increasing or decreasing the available light. In the notes you can see what calculations you need to do if you increase the ISO and / or add on a light modifier (this is not an extensive list please leave a message as you learn what other modifier do and I will update the table).
The tables can be used to calculate both (1) F-Stop you need to set for a given subject to flash distance (2) the subject to flash distance needed for a given F-Stop. The calculations are:
F=GN/D : F-Stop equals the Guide Number divided by Distance (Flash to Subject). In feet
D=GN/F : Distance (Flash to Subject) equals the Guide Number divided by the F-Stop. In feet
The first table shows the Guide Number for a single 580EX II flash
The next table shows the Guide Number for a single 430EX flash
The next table shows the Guide Number for a two 580EX II flash units firing next to each as one unit
The next table shows the Guide Number for a 580EX II plus a 430EX flash unit firing next to each as one unit
The next table shows the Guide Number for two 580EX II plus one 430EX flash unit firing next to each as one unit
The final table shows the multiplication factor needed to calculate the new Guide Number based on any modifications you do to the camera to increase it's sensitivity to light, i.e. increase the ISO setting, or modifications you do to the flash, e.g. add a gel, a sto-fen filter, speed grid etc.
Some simple examples:
Using 1 580EX at F5.6 @50mm to shoot indoors for a portrait.
To get the distance we use the Guide Number for 50mm and divide it by the F-Stop number 5.6.
The best working values for us would then be 1/8 @ 8.7ft or 1/16 @ 6.1ft.
If I wanted to warm up the image with a 1/2 CTO gel we need to reduce the Guide Number by 1/3 of a stop. We would then multiply the distance by 0.87. This would give us the new distance values 1/8 @ 7.6ft or 1/16 @ 5.3ft.
If I were to now add a shoot through umbrella that reduced it a further 2 stops (multiply by 0.5) the values would change to 1/8 @ 3.8ft or 1/16 @ 2.6ft.
I am a metric person, so I use the metric version of these charts which I previously posted.
I have also created a spreadsheet (1.6MB) that contains worksheets with the Guide Numbers pre-calculated for each of the setups mentioned above. There is also a calculator for each of the setups mentioned above which allows you to enter in an F-Stop and it will give you the corresponding flash to subject distance and visa versa. The spreadsheet is 1.6MB and you can download the spreadsheet here.
Please feel free to leave comments or questions below or email them to me.