samueli wrote in post #17495592
Hmm... yn-560 III and yn-560 IV are the same price at Amazon. I could see getting the "IV" version, if I can enable/disable the flash on the one connected to the camera as master, as I see fit. Is that how it works?
Not really. Most folks would get the IV if they wanted the flash on-camera to fire as well, otherwise, they'd just use a YN-560-TX as the on-camera transmitter. Understand, though, that the YN-560-TX can only control the power levels on YN-560III/IV flashes. It can't control the power on a 580EX or 430EX. And, of course, you're manual only and stuck at your sync speed or slower. Also know that the despite the guide numbers, your 580EX is more powerful than the YN-560 models--they're about halfway between a 430EX and 580EX in terms of power.
And YN also makes a 600EX-RT clone. 
And this custom function thing: Do you connect the flash to camera, make change and it stores the change inside the flash?
If you had MkII flashes, you could do it this way, and it's easier to know what you're setting to what with the camera menu. But on the MkIs, you have to change the setting directly on the flash itself.
1. Hold down the backlight/C.Fn button until the C.Fn icon shows up.
2. Press the left/right buttons until the Custom Function you want to change is displayed (01 - 06)
3. Press the SEL/SET button until the display flashes.
4. Press the right/left button to change the setting value (see the chart in Mike's post above).
4. Press SEL/SET to set the value.