Stupid Question Re: Master and Slave Flashes

I'm sure this is a dumb question, but it just hit me today that my current plan for studio lighting may not work.

If I'm using my OCF as slaves triggered by my 580EX II or 7D in Master Mode, but position the slaves behind and above me flashing into an umbrella, are they going to be able to be triggered by my 580? I know the red part of the flash needs to "see" the 580s flash in order to be triggered so if it's behind me and facing away from can it still "see" the flash?

I am planning to get either the Cactus V5 or RF-602 wireless triggers, but not until I moved because they're coming from China and you never know how long that takes and we're moving into a new house in 3 weeks. I'd rather not have them lost between homes.

It depends on how effective surrounding surfaces are at bouncing the signals.

An off-camera ETTL cord can be used to point the 580EXII Master behind you.

I have used a reflector to bend the signal around and behind a post. If you can see from camera position the red "slave light" winking in the reflector, you should have a good line.

Sometimes it's enough to turn the flash in the hotshoe around, to point towards the slaves. This assuming you don't need the master's light elsewhere in the scene.

First, and I may be mistaken, but I don't believe it's the red part that actually transmits and receives; it's another sensor that's right around there.

And based on my own experience using ETTL and infared, the answer is yes, no, maybe, and sometimes. I try to keep my slave flashes at least at 90 degrees to the camera. Typically, my assistant will point the flash body towards the master and the head towards the subject. Even then, performance isn't always 100%.

Slave flashes will still fire if behind the camera/master flash if the space is dark enough and the signal does a good job bouncing around. However, the umbrella could put a stop to even that. I would recommend using silver reflective umbrellas instead of shoot-throughs so that you still get the benefit of the umbrella without blocking the sensor.

FWIW, I just ordered a set of Strato non-ETTL triggers from Phottix. Though the combo works pretty good much of the time, I'm just getting too frustrated with both infared and ETTL. It'll be a pain to meter, but I want the reliability of triggers and the consistency of manual.

For anyone who cares, I got the last package in the mail today and played around. The 580 triggered everything except my 430EX II which held the Beauty Dish. But I think this is because the beauty dish is wider then the lens and it completely covered it so no light was able to reach the flash to trigger it. The YN-560s reflecting into an umbrella went off just fine every time.