Are my 580ex II's strong enough ...

to overpower the sun ?

I have 2 580ex's and I'd like to take pictures where the sky is underexposed to the point of it looking like the pictures were taken at dusk/night - at 3pm.

If they're not strong enough, then I guess I'll have to lug my strobes .. but I figured I'd ask here first

At what distance from subject?

Let's say 2 feet - on either side

If you zoom your 580EX manually to 105mm lens FL coverage, the Guide Number at ISO100 is 190. 190/2 = f/95. Sunny 16 rule says 1/ISO at f/16 is exposure in sun. So you definitely overpower the light; you could equal the sun to 11.9' out! Even with the FL setting set to 50mm coverage, the GN is about 138, so 138/2 = f/70; you could equal the sun to 9.2' out!

WOAH.
Wilt, I appreciate your comment, but I don't understand 1/2 of what you said

I'm going to re-read it a few times and get back to you

Slowly steps away from this thread ..

wait !
I think I get it.

If I set my 580's manually to anything over 50mm - and my ISO to anything over F16 - I should be able to do it.

Sits back in the corner with her dunce cap on..

Wilt is right. But you don't have to take his word for it.

A little experimentation will be revealing.
With your camera in manual mode and the flash turned off, set the shutter speed at flash sync (1/250 on your 30D), ISO 100, and point it at the sky. Adjust the aperture until the meter's needle is centered. Take a shot. The sky will be a nice, deep blue.

Now you can continue to stop down the aperture until the sky is as dark as you want. Try f/16, f/22 and f/32.

When you have reached the setting that gives you the sky exposure you're looking for, turn on the flash and look at the distance scale. That's how far away you can fully illuminate a subject with flash at that aperture. Take some test shots and see if it's right.

Mrs Smarty Pants, here is the derivation of what I wrote...

According to Canon, at ISO 100, at f/1.4 the 580EXII flash can reach as far as 98.4'.
1.4 * 98.4 = 138. So Guide Number is 138.

GN = aperture * distance. So at 2' distance, at full power the flash would require you to set f/70 (aperture = GN / distance).

Or, since the Sunny 16 rule says f/16 for daylight exposure in sunlight, you can shoot as far as 9.2' and equal the sun's light (distance = GN / aperture)

You guys ROCK !!
I totally get it now !!

Thank you so much Curtis & Wilt for slowing it down for me

Wilt wrote in post #3577172
since the Sunny 16 rule says f/16 for daylight exposure in sunlight, you can shoot as far as 9.2' and equal the sun's light (distance = GN / aperture)

Not to quible, but that's at 1/100 shutter speed and ISO 100.

At 1/250 shutter speed she could get to f/10, which would let her flash reach out to 13.8 feet (assuming GN=138 ).