Flash output difference between 430ex and 580ex?

Ok, so I looked at photonotes.org and compared the two, but I can't see what the difference in flash output is or how much more the 580 has v. the 430. I'm confused about how "flash output" is a quantified or measured.

Thinking of upgrading some lenses and flashes and stuff after I get my tax refund. Thanks ya'll!

cowboylife wrote in post #2951369
Ok, so I looked at photonotes.org and compared the two, but I can't see what the difference in flash output is or how much more the 580 has v. the 430. I'm confused about how "flash output" is a quantified or measured.

It can get a little complicated.

The easiest way to compare flash units is to compare guide numbers, being careful that both guide numbers are measured at the same coverage angle (zoom).

When zoomed to 105mm, the GN of the 580EX is 58 meters and the 430EX is 43 meters. The ratio between them is about 1.38, which is a little less than one stop. (one stop is the square root of 2, or 1.41)

So in round numbers, the 580EX will allow you to use a one stop lower ISO, or a one stop narrower aperture than the 430EX in a given situation.

More about guide numbers here.

Look at the Guide Number, but be sure to use comparable FOV coverage angles when flash units have telescoping zoom lenses to increase the long distance capability when telephotos are used.

In the case of Canon flashes, the model number shows the Guide Number in meters. GN43 means when lens is f/2 the flash reaches out to almost 22meters; GN58 means the same f/2 lens has enough light with the flash reaching out to 29meters...about 25' farther, when you compare the GN ratings in feet (141 vs. 190).

Awesome, thanks for the quick replies Curtis and Wilt!!