Canon ETTL & Multiple Flash Units

[Jelster]

My 7D has the wireless flash facility which I am finding really cool, and I want to expand this a little further by adding a second flash unit and potentially a suitable softbox arrangement.

My question is, when using ETTL, does the system realise that the flashes are being diffused by a softbox and diffuser (stofen/umbrella) or do I have to compensate for it in Ev/Flash Ev ?

Steve

 

[Kris]

My question is, when using ETTL, does the system realise that the flashes are being diffused by a softbox and diffuser (stofen/umbrella) or do I have to compensate for it in Ev/Flash Ev ?

Steve

Yes.

No.

 

[vaizki]

The system measures Through The Lens (TTL) by having the flash units emit a pre-flash. This is then metered by the camera and the flash output adjusted to reach the desired exposure.

Thus it doesn't matter at all how you modify the light from the flashes, as the light reflected from the subject is the only thing considered by eTTL. The system doesn't even know how many flashes you have in each group (there is no way for flashes to talk back to the camera).