Sample question: Guide numbers

What is the significance of a guide number?

Buying a flash gun?
The quoted Guide Number effectively shows you how powerful the gun is. It's power determines how far the light can reach, and at what intensity.
Here's an example: A flash with a guide number of 40 (at ISO 100), in feet, will be able to illuminate a subject at 10 feet, using an aperture, (stop), of f4.
How do you work it out?
GN (guide number) divided by distance, equals stop.
Realistically, let's look at the Canon 580ex flashgun. It's quoted as  *190 (at ISO 100, feet) / 58 (meters); flash head at 105mm zoom setting* (Source, Canon USA).
So, breaking this down, we can see that the GN is 190 at ISO 100, at a setting of 105mm.
This is a tad misleading, but still perfectly  true. The output is 190 at 100 ISO, when the gun is zoomed to 105mm, but it rarely will be. The zoom head on the gun is focussing the beam, and therefore making the "reach" a little more than if it were set to 50mm, for example. I'm splitting hairs here though.....
So, armed with your Canon 580ex, at a zoom setting (on the head), of 105mm, using ISO 100, at 10 feet, your aperture will be f19. Too much!!!!! Lucky it's an ETTL gun, and adjusts it's own output!
Looking at a common scenario, you are asked to shoot a small group of people with flash, (maybe at an event).  The group needs to be 20 feet away to get them all in the frame, and they are in two close rows. From front to back the depth of this group is about 4 feet. If you now shoot at full power, (iso 100, GN 190feet), your aperture will be f9.5.  That's enough to ensure a fair depth of people "in focus."   That  though, leads to the consideration of a new problem: Depth of field.  Another day for that one.... It's getting late.
Hope you liked this,
Laurence.