Filipe M.

You had to go and open that can of worms, didn't you?

Well here we go, I'll try to keep it simple:

The Guide Number is a single number measure of a flash's maximum power output, expressed either in meters or feet for a given film speed. A flash with GN36 @ISO100 is more powerful than another one with GN19 @ISO100, for instance (as long as you're comparing the same units). It's derived from this expression:

GN = distance x aperture

What this means is that for a distance of, say, 4 meters and a flash with GN36 @ISO100, a full power pop will give you a correct exposure at f/8 and ISO100. The same way, at 8 meters you need to be at f/4 to obtain the same exposure. In essence it's as simple as this.

And then you get the other variables thrown in...

... like ISO. Just like available light, flash exposure is also affected by ISO. This means that our imaginary flash with GN36 @ISO100 now has a GN50 @ISO200 (the 41% increase Keith was talking about, or multiplying the number by the square root of 2 = approx 1.41)). At ISO400, the GN doubles and is now GN72 (another 41% increase from ISO200). If you plug figures into the above formula you now get 4 meters @ f/16 @ ISO400, for instance.

... and zoom head position. Most add-on flashes come equipped with a fresnel head that allow you to disperse or concentrate the light in a wider or narrower pattern* to match your lenses field of view. What this means is on a wider zoom setting (like 24mm) it'll have to spread the available power to a wider area than on a narrower beam setting like 85 mm. This translates to an effective GN variation, becoming lower at wider settings and higher at narrower beam settings. For instance, our example GN36 @ISO100 flash is only rated for GN36 at the 50mm beam setting. If you go to 24mm, the GN drops to GN26, and at 85mm it'll go up to GN40. Usually 24mm is the wider it'll go on it's own. When you need to go wider, some flashes have a built-in panel you pull over the flash lens to help disperse light even further. Of course having another element in the light path will knock the output down even further, so in our example flash the GN will drop to 14. If you take our example 4 meter distance, we're now talking f/3.5 @ISO100 to get the correct exposure. Go to 8 meters and you'll need f/1.8... or an ISO bump!

Manufacturers usually provide a table with the GN for every zoom setting and power level, and in the films days before TTL became rule and manual was all you had, photographers used to carry these printed tables around and had to refer to them in the field to make sure their exposures were right. TTL kind of negates the need for the tables because the camera will do all the calculations for you, but it's still useful to at least know the standard GN number of your flash so you know if it'll have enough power for any given situation.

Be careful when you compare different flashes though, as manufacturers will try and make their flashes look good in all sorts of ways. With GN it's all too easy to do, and sometimes you need to look at the full GN table to make sure you are comparing the same values. As an example, take these 2 GNs:

GN30 @ ISO100 @ 35mm.
GN50 @ ISO200 @ 50mm.

Which one is the most powerful flash?
While you can convert the ISO rating to match (just multiply / divide by 1.41) you can't do anything about the zoom head position but check the manufacturers table, as they all vary a bit.

And btw, that's the same flash. All values used in this example were taken from the Nikon SB600 manual.


*look at your flash head while you play with the zoom setting and you'll see the bulb come forward at the wide setting and start pulling back into the head as you zoom to narrower beam settings. This is the reason why the Nikon SB900 is a behemoth of a flash: it'll narrow it's pattern to a 200mm coverage, so it really needs a long tube to be able to achieve this.