Max effective range?

I used to think I was good at math till I came across GN's...Now I think I am.......(Hey, Be Nice)

40D, ISO 400, f/2.8, 200mm, 1/250 And a naked 580EX II...(I think that's all the pertinant info)

I know the 580 accomidates the 1.6x crop,and only zooms to 105mm ect, Ect, ECT....Oh my head hurts!

I am only looking for a theoretical "Referance"....Surly somebody knows where how and when to plug all these #'s in.

Why even bother with the math at all? Mount the flash on the camera with the head pointed forward. Set the camera to your operating settings, turn on the flash and then half-press the shutter button. If the flash is in ETTL mode, the distance scale on the flash LCD will show you the working distance range for the settings on the camera. If the flash is in manual mode, the distance scale will show the optimal distance at which the subject will be properly exposed.

If you're going to be be bouncing the flash off the ceiling or wall, then, of course, the distance(s) shown on the LCD (when the head was pointed straight ahead) will be decreased by two or more stops depending on the condition of the ceiling or wall and the distance it is from the flash.

GN / aperture = distance
GN of the 580EX at max. zoom (105mm) is 58 meters at ISO 100.
Doubling the ISO adds one stop (1.4x). Quadrupling the ISO adds two stops (2x).
So at ISO 400 you would use a GN of 116 meters.
116 / 2.8 = 41.4 meters or about 137 feet.

While I agree with Leo that the distance scale is a widely underutilized resource, it only goes to 60 feet so sometimes it's good to know how to do the calculations.

One caveat: The published guide numbers of flash units are generally considered "optomistic". I've never seen a flash unit perform up to specs when measured with a flash meter.