E-TTL and light modifiers

<p>Hello all,</p>

<p>I've been searching high and low for this answer; I've combed Strobist and many other website but can't seem to pin down a definite answer.<br>

I have a few 580 EX I and II's and a couple of Pocket Wizard Flex TT5's. Now, I mainly shoot weddings and to speed things up I usually shoot off camera flash in E-TTL mode if time is tight. I want to gradually start getting more complex with the lighting design in my shots and I'd like to include some light modifiers to the Speedlights, mainly soft boxes or umbrellas.<br>

Now I understand that manual flash is consistent and you always know what you’re going to get etc so please don’t say 'Just use manual'! And I know sometimes the Pocket Wizards can be a bit hit or miss with getting the exact exposure.<br>

As far as I can tell the EOS system fires a pre flash in E-TTL mode to determine the correct amount of power/light which is needed to illuminate the scene. The second flash then delivers this power based on the pre flash calculations.<br>

So if I stick a soft box in front of a Speedlight should the E-TTL system compensate for it? What about distance from the light to the subject? Meaning if i want a wider shot and I have to move the light to keep it out of the cameras field of view and that light got moved 5, 10 ft away would the E-TTL system still be about to calculate the correct exposure for that shot? If no, why not? Is the light sensor on the flash covered? Are there anyways around this?<br>

If anyone knows of a website or can point me in the direction of an information that is related to this I would be very grateful.<br>

Thank you in advance.</p>

<p>E-TTL records the light reflected back to the camera from a test fire. So it by definition compensates for any modifiers you have in the light path. E-TTL flash units do not employ any light sensors at all - the metering is all done by the camera.<br>

http://photonotes.org/articles/eos-flash/</p>

<p>Just to answer what might be the next question ("But isn't that thing on the front of the flash a sensor?"), there are a couple of answers. Both generations of the 580EX (along with all 400-series EX flashes) have a sensor on the front that's used only for communication when using Canon's wireless multiple flash system; this sensor has nothing to do with metering. The 580EX II also has a light sensor on it, but that's only used when you set the flash unit to do its own metering (the way auto flash units from the 1970s work); if you're doing that, you're not using E-TTL.</p>

<p>http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=946<br>

I havent read the whole page of this but it looks as though it has.</p>

<p>ignore may last!</p>

100 percent the auto system will take all modifiers and varying subject distances into account, the reason people run into

problems and subjects get under exposed is that once you are in the softbox you lose one stop of power, minimum, that is

half your output. Often the head is firing at full power but it is not enough, hence the apparent underexposure in ETTL II.