Quote Originally Posted by Chuck Lee


So, inorder to balance the two Mark will need to do a little calculating. So the 430 has 43/58 the power of the 580? Is that correct?


Rounded out with about a 1/3stop margin of error and I'd say 2/3 the power. Correct? So, to get a 2:1 ratioof the 430 to the 580 (that would be the 430 used as the key) Mark would need to set the 430 at full and the 580 to 1/4 power. Oh, but wait, We haven't accounted for the subject distance. Dang, this is going to get confusing.


I think/suspect he's going to work E-TTL since he's constantly mobile. In that case, the camera meter will see the intensity of the A group (i.e. the 580 on-camera) and the intensity of the B group (i.e. the 430 off-camera), figure out how much variation is needed to achieve a 1:4 ratio, and command that in time for the actual exposure. A 1:4 ratio means the 430 will be asked to provide 4x as much (metered) light, but it might be closer to the subjects so it's all a guess.


Think of it this way: you'll wish the 430 would beep when ready again. If it's ready (the focus assist lamp will blink when in slave mode and ready) in <=2 seconds, you're probably at or below half-power, and that's a good thing.