Flash Guide Numbers

Flash Power and Guide Numbers

The flash guide number (GN) is a useful indicator of the power of a speedlite. In general the larger the GN number the more powerful the flash but this isn’t always the case as in order to compare two speedlites the parameters have to be the same (i.e. full power, ISO ISO and the same focal length, 35mm is used as the standard)

The mathematical formula used to calculate the Guide Number (GN) is given as:

Guide Number = [distance from flash to subject] x [F-stop]

In their marketing material manufacturers usually only give us the speedlite’s maximum GN number. This is the GN number measured at the longest focal length and is usually at ISO 100.

For example the maximum guide number for the Canon 600EX II-RT is 60m (60m@200mm & ISO 100). This is measured when the flash is at full power 1/1.

Since Guide Number = [distance from flash to subject] x [F-stop]

and GN = 60m, if we use an F-stop of f/4 then the distance from flash to subject needs to be 60/4 = 15m for a correct exposure. If we used and F-stop of f/8 then the distance from flash to subject would be 60/8 = 7.5m. Easy!

Beware of marketing hype.

The Canon 600EX II-RT has a high GN number not because it is super powerful (although it is one of the best for its size) but because it has the ability to illuminate a subject a long way away. Obviously raw power has something to do with this but it’s also because the speedlite can focus the light emitted from the flash head into a narrow beam (it has a long focal length of 200mm). If the GN number was measured at a focal length of 35mm then the light beam would be much wider and the guide number would be much less. Canon give a guide number of 36m@ 35mm & ISO 100.

The guide number depends on the zoom setting (focal length) of the speedlite as this controls how the light spreads out. Manufacturers often include a table of rated guide numbers as various focal lengths in the back of the operator’s manual. This allows you to compare the power output of various speedlites at the same focal lengths which is a much more accurate way to compare the power of two speedlites.

Canon 600EX ii-RT table of guide numbers at various zoom and power settings.
Extract from Canon 600EX II-RT manual showing guide numbers at various zoom and power settings. This allows you to calculate the correct flash to subject distance for a given f-stop, or you can calculate the correct f-stop for a given distance.

In our speedlite reviews we have included the speedlites maximum guide number taken at their longest zoom setting but also the guide number taken at 35mm to allow comparisons to be made.

Canon Speedlites listed in order of power

Canon 600EX -RT (GN 36m)
Comparative Guide Number 36m @ 35mm & ISO 100
Maximum Guide Number 60m @ 200mm & ISO 100
Read Review

Canon 580EX ii (GN 36m)
Comparative Guide Number 36m @ 35mm & ISO 100
Maximum Guide Number 58m @ 105mm & ISO 100
Read Review

Canon 600EX ii-RT (GN 34m)
Comparative Guide Number 34m @ 35mm & ISO 100
Maximum Guide Number 60m @ 200mm & ISO 100
Read Review

Canon 470EX-AI (GN 29m)
Comparative Guide Number 29m @ 35mm & ISO 100
Maximum Guide Number 47m @ 105mm & ISO 100
Read Review

Canon 430EX iii-RT (GN 28m)
Comparative Guide Number 28m @ 35mm & ISO 100
Maximum Guide Number 43m @ 105mm & ISO 100
Read Review

Canon 430EX ii (GN 28m)
Comparative Guide Number 28m @ 35mm & ISO 100
Maximum Guide Number 43m @ 105mm & ISO 100
Read Review

Go Compare!

Let’s compare the 600EX ii-RT (GN34m) to the 430EX iii-RT (GN28m)

Guide Number (GN) = distance (m) x F-stop
or
F-stop = GN/distance (m)

Therefore at a distance of 10m
F-stop (600EX ii-RT) = 34/10 = 3.4 (nearest 1/3 stop is f/3.5)
F-stop (430EX iii-RT) = 28/10 = 2.8

Therefore if I set my speedlite zoom head to 35mm and put my speed light at maximum power and then placed my subject at 10m away from the speedlite, for the Canon 600EX ii-RT I’d need to set my aperture at f/3.5 to get a correct exposure. If I used the Canon 430EX iii-RT I’d need to use a slightly larger aperture of f/2.8.

So how do we calculate the difference between f/3.5 and f/2.8. Most cameras allow you to increase the f-stop in increments of 1/3 stop. So you could just check this on your camera f/3.5 → f/3.2 → f/2.8 is two clicks so 2/3 of a stop difference. Alternatively you could use the table below which lists the f-stops in 1/3 increments.

Table showing F-stops listed in full stops and 1/3 increments

Therefore the difference in power output at a zoom setting of 35mm between the Canon Speedlite 600EX ii-RT and its smaller sibling Canon Speedlite 430EX iii-RT is about two-thirds of an F-stop (the 600EX ii-RT is about 1.66 times more powerful)

If a lighting set up required f/8 using the 600EX ii-RT then the same lighting set up using the 430EX iii-RT would require an equivalent setting of f/6.3 – certainly not a deal breaker in most portrait & studio situations especially when using soft boxes. However the 600EX ii-RT has a great advantage at longer focal lengths in the 105-200mm range when used without any modifiers. Such situations may include weddings, events, or even paparazzi where you need to be a bit further away or you are trying to get the background out of focus.

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