That white background - AGAIN

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Picture the scene:
A small dining room studio set up, with 2 Elinchrom D-LITE 4 softboxes, working off a radio trigger (top mounted on a 5Dii), with a single sensor at the rear of one of the blocks on the power input picking up the signal, and the second elinchrom block selected to 'slave' on the rear panel. This illuminates the subjects fine - no issues.
I have a white background roll situated some 2 - 3ft behind the subjects, and understand the fall-off will make it grey (which it does rather well).
I also have a 580EX and a 580EXII - what do I need to simply have these 2 flashguns light the background only?
Would a simple 'sensor cube', individually mounted between the flashgun body and the flashgun foot on each gun pick up the light and blip the background?
Do the guns need to be set manually or ETTL?
Open to advice, guidance and experience.

[Cutter]

I would use your 2 Elinchrom D-LITE 4 softboxes and their in built sensors to light the background and get a nice even light. Making sure its a coulple of stops lighter than your subject. Then use your trigger on your 580s with softboxes to light your subjects. The 580s will trigger the sensor on your 2 Elinchrom D-LITE 4 softboxes ( I think)

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Cutter
Many thanks for the reply.
I like the idea, but sadly the receiver for the Elinchrom is an in-line connection from the wall power cable to the back of the flash block only - hence ideal for the 2 Elinchrom, but no way to fit for the 580.
I do have an Canon OCF cord to put one 580 about 1m away from the camera - a flash sensor may trigger the other 580 off. Sadly, I then lose the trigger function to the master Elinchrom as the OCF cord fits into the camera hotshoe...
Grrr

[cosmicma]

bit late on this reply

you could allways use one of these or similar

http://www.jessops.com/online.store/...9094/Show.html

this will allow you to fire the 580's with the main D Lites
you will have to set the 580's manually but that shouldn't be too much trouble