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.

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## 4 comments:

Please please help! How do get the guide numbers for the combination of two 430ex's? Thank you so much by the way. This article opened a whole new world for me.

You calculate two devices by using the square root of the original guide number. The calculation looks like this:

Square Root((Guide Number x Guide Number)+(Guide Number * Guide Number))

So the guide number for for 1/1 Power @14mm is 51.05. Calculated by doing the following

=Square root ((36.1 x 36.1)+(36.1 x 36.1))

You can use the same formula to calculate the guide number for two flashes for any setting.

Hope this helps.

Cheers/Peter

Thank you so much for your response. However, I don't get it! My numbers are coming out huge! I'm not understanding it at all. When I combine two flashes, I don't just add the guide numbers right? Obviously not, but that's my thought. Please help again. I'm a little slow. Thank you so much!

The problem you are facing is due to the way you are calculating the Guide Number.

Instead of simply adding up the two guide numbers, e.g. 36.1 + 36.1 = 72.2 which is wrong, you must use a more complicated formula using a square root calculation that I mentioned above.

The calculation looks like this:

The guide number for two flash units = Square Root((Guide Number of 1st flash unit x Guide Number of 1st flash unit)+(Guide Number of 2nd flash unit x Guide Number of 2nd flash unit))

So if I took two 430EX II units and put them together and set them both to 1/4 power @ 50mm I need to first look up the chart in the article above and find the guide number for a single unit for those settings is 17.

I then need to use the calculation above to get the following formula

GN of two 430EX II units = square root ((17*17)+(17*17)) which gives a guide number of 24.04

So what if I had two 430EX II units and put them together with two different settings like this:

430EX II Unit 1: 1/4 power @ 50mm

430EX II Unit 2: 1/2 power @ 105mm

You first have to find the guide number for the two units for their respective settings from the chart above. This gives you the following values:

430EX II Unit 1: GN=17

430EX II Unit 2: GN=30.4

So the formula looks like this:

GN of two 430EX II units with two different settings = square root ((17*17)+(30.4*30.4)) which gives a guide number of 34.83

Feel free to ask anymore questions or you can email directly at emmett.photography@gmail.com

Cheers/Peter

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