Thursday, May 22, 2008

Guide Numbers 101: Canon 580EX II and 430EX (Meters)

UPDATE: I have republished this article using feet in the charts below

After reading the book Light Science & Magic and reading David Hobby's Lighting101 and Lighting102 courses at, 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 meters

D=GN/F : Distance (Flash to Subject) equals the Guide Number divided by the F-Stop. In meters

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 @ 2.6m or 1/16 @ 1.9 m.

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 @ 2.3m or 1/16 @ 1.6 m.

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 @ 1.14m or 1/16 @ 0.8 m.

I am a metric person, however if you would like I can also post this article for feet if you request it. Please send me an email if you would like the above tables in feet.

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.

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JJ said...

Thanks for the table Peter, very helpful.

I measured my flash @ 1/8 using a light meter at my max sync speed (1/200 Canon) and noted them down on a "Cheat sheet". But your way seems easier.

Emmett Photography said...


Thanks for the comments. I've had a lot of people complaining that I'm wasting my time with the whole GN thing and that I should go by my instincts, but I think it's more because the GN's are slightly intimidating.

I can definitely say that I have gone from taking about 30mins to get something half decent to about 1 min to getting pretty close.

I'm thinking about trying to work out a real GN for the two flashes and then putting these number in to test so that I could get even closer. Just not sure how to go about it yet. Maybe by the time I've figured out how, I'll be familiar with my equipment and won't need them anymore :-)


buccino said...

hi peter,

with all your effort in doing the GN thing, i still can't understand the table.

it's prolly me though, and i guess you have to really, really, really, explain these to us like we're 5 year olds on the strobist ph meet-up.

i keep on testing my flash (it's a 540EZ canon speedlite and it's not synced with E-TTL so it's always manual for me when i hook it up with my DSLR). i got a 160 as GN (10ft x f16 at 50mm flash zoom spread).

but anyhow, i'm sure you'll explain this to all of us. haha.


Emmett Photography said...


I look forward to meeting up and discussing Guide Numbers. I think that once we go through this topic it should be a bit easier to use.

I found an article for the Canon 540EZ flash (

It has a set of tables for the full range of zooms and output power. These are in meters so you can multiply the number by 3.2808 to get the number in feet.

According to the table the guide number for full power at 50mm zoom would be 137.7. If you use the table figures and the spreadsheet you should be able to get the GN tables for you flash.

See you on Saturday.


kelmadics said...

i am getting really confused of your chart. i thought the GN for example for 580EX ii is 190. how are you able to calculate all this GN in your chart? i thought its a constant Guide Number that would not change.
So now for example im using 50mm my subject is 2 meters away how do i use your chart to plugin the correct FE

Emmett Photography said...

The chart you are looking at is in meters, but the figure you are quoting is in feet.

Using the charts (which are in meters) you would use the formula: F=GN/D.

You would have to decide what settings you want to use flash on, e.g. 70mm @ 1/16, which is gives you a guide number of 12.5.

This would give you the following settings:
F= 12.5/2

You would then set your F-Stop to around 6 (6.3) and your ISO @100 and you should be very close to the ball mark.

If you want to look at the chart in feet you can see it here:

Hope this helps. Let me know if you have any other questions.

kelmadics said...

ahh that makes more sense now. Sorry im a newbie. I didn't realize that you will need to handpick or decide the flash output power. I have no idea what output power should i base my flash in. im guessing indoors is 1/8? When should i use 1/1 or 1/64?

On the side, what should i do in this situation: Outdoors, want to use f/16 for sunset landscape and the subject is backlit and i want a fill in flash that has no harsh lighting. Should i use direct flash or 75(or XX) deg with diffuser or 75(or XX) deg with white card reflector on a 580ex ii? What would you do in this situation if you have nothing but the 580ex and camera?

andy peat said...

Thanks for taking time to share these Peter, I have downloaded the chart and it looks very good to me. I have a question for you that I've been pondering. I am new to off camera flash so dipping my toe in the water but no stranger to photography. Re: the inverse sq. law, I'm thinking that I'll already have an idea of the aperture I need (for the DoF), also a rough idea of Exposure (usually 1/200th sec) so that leaves the flash output to work out (or GN) hence GN = Dist x f.
As I said I'm new to this but that seems to me to be the logical way of calculating but I don't very often see the calculation done this way.
AS an example, say if my distance was 6ft, f stop was 5.6 that gives a GN of 33.6, then I could work that out as a fraction of my flash GN so 580ex is 58. This makes a fraction which is impossible to reduce down as it's 33/58 so I suppose that is why it's never used. Am I over complicating things? I just figure that flash output should be the unknown that photogs should be trying to calculate as Fstop can be used as a creative tool and exposure id fairly limited (flash sync speed at one end and motion blur at the other) Best Regards
Andrew Peat

andy peat said...

EDIT: Please ignore the part about exposure playing a part in this. I should have said distance from light source to subject. this has a huge effect on the look of an image as does aperture.
Hope I've not confused you there.

macmansam said...

So what do I do if I'm bouncing flash out of an umbrella?

macmansam said...

What do I multiply the total by if I'm bouncing out of an umbrella? thanks.

Emmett Photography said...

macmansam, if you look in the chart above you can see that for a shoot through umbrella or a stofen filter you multiply the guide number by 0.5 and 0.41 respectively.

I'm not sure what the exact number is for a reflective umbrella but the supplier should be able to tell you the number of stops it reduces the light by. You can then use the chart above to find the stop reduction and use the factor to multiply the guide number against.

Hope this helps.