I originally posted an article on using Guide Numbers for Canon flashes for both the metric system, but after a few requests I am republishing the article using feet.
These articles used the original Guide Numbers given by Canon in the user manuals. However after a lot more research and time spent looking into this, along with my newly purchased Sekonic Flashmate L-308S, I have come up with a much more realistic set of guide numbers to aid people in getting a much better exposure first time.
Just to re-cap why I did this (apart from being a little bit mad):
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 2 tables represent the Guide Number for the combination of the flashes listed in both Meters and Feet. The third 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 table below shows the "more realistic" Guide Numbers in Meters for the combination of flashes
The table below shows the "more realistic" Guide Numbers in Feet for the combination of flashes
The multiplication factor to change the Guide Number
You can download the Excel Spreadsheets for both Meters and Feet for the above tables.
Now I appreciate that some people may feel either cheated by the original guide numbers (how do they calculate them?) or may feel that taking a whole Stop off the guide number makes the whole image too hot (I don't but some people will). So the 2 tables below the Guide Numbers for the combination of flashes with 2/3 Stop off.
The table below shows the Guide Numbers in Meters with -2/3 Stop for the combination of flashes
The table below shows the Guide Numbers in Feet with -2/3 Stop for the combination of flashes
And for those of you who just can't believe that Canon may have overstated their number just ever so slightly (1 Stop!) here's the original Guide Number tables
The table below shows the Original Guide Numbers in Meters for the combination of flashes
The table below shows the Original Guide Numbers in Feet for the combination of flashes
Well I swear that's the last article on Guide Numbers - except maybe if I get some more flashes :-)
As ever please feel free to leave comments or questions below or email them to me.