Canon Master/Slave Questions

[bazooka]

So far I've only used Manual lights but I've got a project coming up that requires me to do 175 somewhat environmental portraits, each in a different location, and they need to be taken fairly quickly. I figured it would be worth it for me to buy another E-TTL flash so I can have the key off-camera. So I have my 40D, a 550EX, and a 580EXII.

First question is this... if I put either flash on the hotshoe, set it as master, and turn off the "flash" setting so it should only send a flash signal to the slave (the slave flash is turned off to simplify the experiment) before the shutter opens.... the flash is still partially exposing my picture. At f/4 ISO 200 in a dark room, it's powerful enough to properly expose a subject two feet away. Where is this light coming from?? Is something wrong with my camera that would allow the preflash to be seen by the sensor?

Second question... how do I separately control the power of the hotshoe master and any one group? (Yes, I read the manual. I found it to be poorly written and didn't explain much.) Do I use the FE on the master to control the master? If so, how do I control the FE on any given group?

Third question... when I have the 580EXII on the hotshoe as master and the 550EX as slave, a lot of times the 550EX will only fire the preflash (I can see it out of the corner of my eye) but the photo shows that it did not flash during the exposure, but occasionally it will fire at full power, completely blowing out the subject. When I use the 550EX as master, the 580EXII as slave seems to meter and exposure correctly everytime. What are the possible reasons for this? Is it possible the 550EX is faulty?

Thanks!

 

[bazooka]

Ok, I think I found the answer to #1.... the flash I'm seeing is not the metering flash but the trigger flash which tells the slaves that the shutter is open and they need to fire. So it seems if you're shooting in the dark and you want perfect black in the shadows, E-TTL is not the way to go?

I'll keep looking for answers for #2 and #3 if someone doesn't beat me to it.

 

[bazooka]

Answer to #2... Master is always in group A. If I want to adjust Master and another flash with a ratio, I need to set that other flash to group B and use ratios.

The more I read though, the more I think there is something wrong with my 550EX. When hitting pilot on the 580EXII, the 550EX will not fire.

 

[Big Mike]

Ok, I think I found the answer to #1.... the flash I'm seeing is not the metering flash but the trigger flash which tells the slaves that the shutter is open and they need to fire. So it seems if you're shooting in the dark and you want perfect black in the shadows, E-TTL is not the way to go?

As far as I understand, the Master flash may use a small series of flashes to communicate with the slaves, and of course there is the E-TTL preflash. So all of these will fire very quickly, right before the exposure is open, and then the actual flash will fire when the shutter is open.
If you set the master flash to 'does not fire', you shouldn't get any light from it in the photo. There is no 'trigger flash' that fires after the shutter is open.

Answer to #2... Master is always in group A. If I want to adjust Master and another flash with a ratio, I need to set that other flash to group B and use ratios.

Yes, I think that's correct.

The more I read though, the more I think there is something wrong with my 550EX. When hitting pilot on the 580EXII, the 550EX will not fire.

Maybe that is just to fire a test flash, and you're assuming that the 'slave' has an optical trigger. This system doesn't work via optical triggers the way that studio lights (or simple optical triggers) work. For the Master flash to trigger the slaves, I think you have to actually fire the whole shebang (just take a test shot).