I'm a new owner of a 550EX with plans for another and a ST-E2 transmitter. My visions of q&d (quick & dirty) E-TTL w/multiple flashes is clouding my thinking. As a result, I don't know what slave group is. Being an ElanIIe user, I am restricted to one slave group only. Once again, Canon's excuse for a manual is far from in depth. So I turn to you, my fine EOS friends, for help.

In my own defense, I was thinking to achieve a flash ratio with the slave, I could set the slave to manual and try different output settings. 1/2, 1/4 or whatever. Is this possible?

Thanks.

-- Colin Miller (ckmiller@pond.net), July 23, 2000

Answers

To get a flash ratio with 2 ttl (or e-ttl) flashes all you have to do is vary the distance of each flash from the subject. Example: left flash is 5 and a 1/2 feet (f5.6) from the subject and right flash is 8 feet (f8) from the subject you should achieve a one stop difference. I've done this from time to time and it works fine but I do verify the difference with a flash meter to be sure. If you use manual settings you WILL have to use a flash meter to make it work correctly.

Hope this helps.

-- Paul Nicol (nicol@roadrunner.nf.net), July 23, 2000.

I was under the impression that varying the distance of flash to subject will not change anything but the output (in w/s) of the flash. Example: flash is 8 feet away and camera is set at f/5.6. E-TTL will require the flash to produce enough light for an f/5.6 exposure regardless of where the flash is. At 4 feet, the flash will produce much less light as subject is half the distance as previously. 8, 10 or 20 feet shouldn't matter. That creates a problem if I want to move the flash around for less light output only.

-- Colin Miller (ckmiller@pond.net), July 23, 2000.

Sorry, it's the way that I explained it. The both flashes will put out the same amount of light but because the right one is 2.5 feet back from the subject it will be about a stop less light hitting the subject. If the subject is a person, it won't matter where you meter for flash, there will always be a difference of about one stop from left to right. The flash closest to the subject would be your dominant light.

-- Paul Nicol (nicol@roadrunner.nf.net), July 23, 2000.

Paul's right. Two 550EX's driven by an Elan II don't allow multiple groups so you can't set ratios on camera or on flash. This does work with the EOS 3 or 1V though. Anyway, moving the flash units different distances will create ratios because both flashes are controlled together and will put out the same power, even though one is farther from the subject.

-- Jim Strutz (jimstrutz@juno.com), July 24, 2000.

Following up on Jim's post: even the marvelous master/slave ratio control (from 1:8 to 8:1) on the EOS 3-550EX combo can be altered and messed up by placing flashes at different distances from the subject. I've learned it the hard way (well, not really, I was smart enough to run a test roll and recommend that you do the same with various flash- subject distance settings). Piotr

-- Piotr Mikolajczyk (mikolaj@pcwarsaw.waw.pl), July 24, 2000.

How about setting both 550EX's at the same distance, and using the built-in diffuser of one of them to effectively reduce its output by a stop? This would make the non-diffused flash the main light.

SteveB

-- Steve Bromley (sbrom@austin.rr.com), August 13, 2000.

I wonder if Piotr could elaborate on his comment about being able to mess up the ratio by varying the distance.

I thought the whole purpose of the ratios was so you didn't have to worry about distance.

Paul

-- Paul Ferrara (paul@columbusoft.com), November 09, 2000.