ETTL with off camera flash

This scenerio employs an on camera flash (ie 580 EX II) set to ETTL, with one or more off camera flashes.

The control in this scenerio is canon infrared.

Ok the sequence is as follows:

Focus, shoot, preflash, reading by on camera flash, and then a true flash at the power determined by the preflash.

This preflash has no communication with the other off camera flashes.
The on camera flash has no idea how may off camera flashes there are.
The on camera flash has no idea how far the off camera flashes are from the subject.
The on camera flash has no idea of what type of diffusers are on the flashes

Each external flash will receive the same information, and that information will tell the slave to fire at the same power as the master on camera flash.

The result can be over exposure if this not anticipated. The exposure is multiplied by the number of flashes.

With the 580s set to groups you can increase or decrease the groups exposure -3...+3

The slaves can also be set to manual, and this is what I traditionally do.
Some times it is just as easy to move the slaves (if on a stand or held by an assistant) closer or further away.

My understanding of the 600 EX RT system is that when used with the 5D III or 1Dx that you may be able to set up groups and have the groups communicate such that the preflash reading can be divided and not multiplied. Not sure if this is any kind of advantage as this system will assume that each external flash is the same distance as the master from the subject.

That being said I think I will continue to use my slaves in the manual mode and either change their setting from the camera, the master flash, or simply moving the light stand or my assistant.


Maybe that is why there is just so much trial and error in my set ups.
Thanks for digital.

Remember (if you are old enough) when you bought film and had to decide between 24 or 36 shots, and had to wait a week to have the photos developed.

We should really expect only the best with the equipment we have today.

But still I look back at some the the older photography and wonder how they did it.

Comments? Corrections? Better Explanations?