How does off-camera ETTL even work?

Isn't a major component of ETTL that the flash assumes it is ON the camera? If you use off camera ETTL with radio poppers (for example) and the flash is not near the camera, how the heck does it work. I have to be missing some huge component of this sort of setup.

While I don't have a technically details answer, my limited understanding is that it's all in the pre-flashes (the stuff the human eye doesn't see).

That, or magic. That must be it - magic

Why does flash assume it is on camera? You can use eTTL cord and move the flash away from camera, isn't it? So with radio popper or newer PW, it is same thing just the cord got replaced.

See, I didn't even know that. So it pre-flashes, measures the light and then set's itself?

imahawki wrote in post #10659746
See, I didn't even know that. So it pre-flashes, measures the light and then set's itself?

Correct.

Chris

The flash pre-flashes, and the camera measures the light.

OK, so the reason I was asking is I'm thinking about going back to flashes rather than studio strobes. I often take my kids on outings for some planned photos and usually just want one light source for fill. Lugging out Genesis and Vagabond II is getting to be a little much and it means I don't feel as "nimble" if that makes sense. But I'm checking out the pocket wizard and radio popper web sites and my head is swimming. What should I look at that actually walks through using each of these products with say 1 or 2 flashes, both off camera. Note that a primary goal would be to be use ETTL and change settings from the camera. I've already go Cactus triggers if I want to go full manual and they have never failed me.

I'm in the same position you are - I have Genesis lights and am moving to speedlights and playing with ETTL mode.

I'm lucky, with a 7D I can remotely control them (with line of site - unreliable outdoors) - but I want something more reliable. You could put your 580exII on top of your 50d, and remotely control another compatible speedlight that way to start out.

Wireless option -

Pocket Wizards are an expensive - but professional reliability option.
Pixel Knight triggers from ebay seem to be much more affordable, but early bugs are keeping me from pulling the trigger.

There are others, but I've not researched them yet.

I'll probably be ordering Pixel Knights in the near future.

The only thing that the wireless triggers do is change the signal from optical to radio so that you get better reliability and range outdoors. The flash works the same as if you just are using a E-TTL II system.

I think the other bugaboo with my plans is not understanding how powerful say a 580EXII is relative to a studio strobe. It looks like it would cost about $420 to remotely fire ONE strobe (assuming you already own it) and another $200 for each strobe after that. You gain high speed sync but you are paying a lot of money for a lot less power, right?

imahawki wrote in post #10660494
I think the other bugaboo with my plans is not understanding how powerful say a 580EXII is relative to a studio strobe. It looks like it would cost about $420 to remotely fire ONE strobe (assuming you already own it) and another $200 for each strobe after that. You gain high speed sync but you are paying a lot of money for a lot less power, right?

Yes.

*but*

It's usually enough power. Plus it's considerably smaller. And more portable. And easier to put in odd locations, gel, switch batteries on - etc.

My 580exII has proven powerful enough for anything I've needed it for to date ...

Thank you for the practical response!

imahawki wrote in post #10661179
Thank you for the practical response!

If you need the power of a full strobe, thats what a battery pack (like a vagabond), a bigger backpack, and an appointment with your chiropractor is for

Right... but if I go in this direction I'm planning on selling the Genesis(es) and Vagabond.

imahawki wrote in post #10661193
Right... but if I go in this direction I'm planning on selling the Genesis(es) and Vagabond.

Me too

My end goal is to sell on the Genesis lights and fund a second 580exII, new stands (using the Calumet ones now) and a second QBox 24 softbox.