Omit the trailing 0 in any Canon speedlite flash model and that's the flash "guide number" when measured in meters. e.g. the 320EX has a guide number of 32. The 430EX II has a guide number of 43.
The "guide number" is the distance that the flash can adequately illuminate a subject assuming (a) ISO 100 and (b) f/1.0 aperture. The ISO is no problem, but the f-stop can be a bit confusion. The industry standard is to use f/1.0 BECAUSE it makes the actual math easy. Just divide the guide number by the f-stop you actually plan to use.
43 meters works out to about 141 feet. Suppose you're shooting at f/5.6, then: 141 ÷ 5.6 = 25 feet. So the flash can illuminate a subject adequate at 25 feet if you use ISO 100 and f/5.6. Of course you can increase this distance by bumping up the ISO... increase the distance by a factor of 1.4x for each stop of ISO you add.
It's nice to have a bit more power because the flash looks better when you bounce it or use lighting modifiers (which tend to eat a bit of the light.)
Incidentally... the Speedlite 430EX II does have a built-in focus-assist beam which allows your camera to focus in extremely dark situations (it's a red LED which projects a pattern that the camera can use to accurately lock focus.)
Tim Campbell
5D III, 5D IV, 60Da
