Canon’s speedlite model numbers include the “guide number” (in meters). E.g. the flagship 600EX flashes have a guide number of “60” . The 430EX flashes have a guide number of “43”. The 320EX flash has a guide number of “32”.
You can see the pattern... drop the letters as well as the trailing zero and what remains is the guide number.
So what’s a guide number?
It’s sort of an indication of power output but that’s not really accurate. It’s an indication of how far away the flash can properly illuminate a subject based on the assumptions of ISO 100 and f/1.0. But most flashes have an adjustable (often it automatically adjusts itself) reflector and that reflector can shape the beam causing it to concentrate the light to better illuminate subjects at a greater distance (so it’s possible to get more light to go farther without actually increasing the power of the light). But technically the flash does a combination of both (adjusting the reflector to shape the beam AND adjusting the power output).
Guide numbers are sometimes in meters and sometimes in feet. But Canon uses meters.
You wont ever actually shoot a subject at f/1.0... but the number is used because it makes the math easy. It turns out if you simply divide the guide number by the ACTUAL f-stop you plan to use... the math works out perfectly (but only if the guide number is based on f/1). Suppose you use the 320 (guide number is 32 meters) and f/5.6. Divide 32 ÷ 5.6 = 5.7 (but that’s meters). Convert the meters to feet and it works out to 18.7 feet.
While 18 feet might seem like a lot... trouble happens when you start to work out ways to improve the light... getting the flash off the camera and shooting through something to enlarge and diffuse the light (which might cut the light output by half) or bouncing the light off a white ceiling or wall (which not only cuts the light again... but also increases the distance).
You can quickly see how the light is getting eaten up to the point that subjects need to be fairly close.
You can bump up the ISO on the camera (running a longer shutter exposure doesn’t help with flash because the flash is only illuminated for a very brief moment.)
Canon’s flashes are, of course, compatible with their cameras and their E-TTL works great (E-TTL is a fairly clever system that helps you get a proper exposure via the flash “most” of the time... it’s not foolproof but it is pretty good.)
There are 3rd party flashes that are considerably less expensive ... with varying capabilities. Some of these do purport to be Canon E-TTL compatible but there’s a broad spectrum of capabilities in the Canon E-TTL system. I would not necessarily expect “everything” to work.
To learn more about flash photography, I’d suggest a few resources:
1 - Speedlighter’s Handbook (by Syl Arena). I think a new edition is due out soon. The current edition is ludicrously expensive to find in “print” edition. It’s reasonably priced on the Apple “iBook” store (and that’s the ONLY place I can find where it’s reasonably priced). Those that still have paper editions are charging “opportunistic” pricing based on the fact that the print version is really hard to find and for some reason it isn’t available on Kindle (I don’t think he released it on anything other than iBooks.). It is easily the best book on the Canon flash system.
2 - The Strobist blog... strobist.blogspot.com. This isn’t a “book”... it’s a blog. But there are loads of helpful articles.
3 - Light, Science, & Magic (by Fil Hunter et. al.). This is a book and is currently in it’s 5th edition. It’s probably THE best general book on the topic of lighting. It isn’t Canon-specific, nor is it about speedlight flash units. It’s really more about the actual “light” and not the thing that made the light.
Syl’s book (#1 on the list) goes beyond just the technical explanations and also gets into several practical use applications (e.g. how to do portraits with just one speedlite... how to do portraits if you have two speedlites... how to do action photography with speedlites... etc.)
Tim Campbell
5D III, 5D IV, 60Da
