If you own a Canon Speedlite 550EX, 580EX or 580EXII you can use any of them as Master controllers in a multi-flash setup. In addition to controlling identical models, they can also control the 420EX, 430EX and 430EX II models.
NOTE: the 220EX and 270EX do NOT have wireless capability therefore they can't be used as slaves.
Personally, I would prefer to use an ST-E2 as a Master controller since it's lighter but, in contrast any of those Master flash units will allow control of three slave flash groups VS the two groups of the ST-E2. So, the Master flash will be heavier and bigger than an ST-E2 but, will allow a little more control.
NOTE: the following was written for the Canon Speedlite 550EX - if you have a 580EX or 580EX II you will have a different interface (a wheel instead of the buttons) but, the general principles should apply. As stated earlier, the 550EX, 580EX and 580EXII are the ONLY units that can work as MASTERS.
SETTING YOUR MASTER AND SLAVE UNITS
The first thing you do is set the unit to Master. If you do not wish its flash head to fire you can turn that OFF - if you do that, the unit will work as a controller and will not fire its own head. This is useful if you only want the remote unit(s) to light the subject without any light coming from the on-camera flash. To turn the flash OFF press teh SET/SEL button until the little flash icon blinks, now push the +/- button to turn it ON/OFF.
You now must choose a Slave channel. This is to prevent your Master flash from firing other photographers' slave units. ALL flash your EX flash units must be set to the same channel in order to communicate. Press the SET/SEL button until your reach the CH option and then, use the +/- to select a number 1-4. Now, it's time to choose a group which Canon calls Slave ID (A, B or C). If you only have one slave flash simply choose A. Again, use the SET/SEL button until you see the word Slave and choose A.
Set your remote (slave) units to SLAVE making sure they are on the same channel and slave id as set on the master and you are ready to go.
If you wish to use different GROUPS (Slave IDs) you must assign each unit to its own GROUP. A Master flash can control up to three groups (Slave IDs): A, B and C. Each group can be given a different exposure compensation and, each of the flash units within a group can also be further modified by setting its own exposure compensation, directly on the unit itself. Finally, you can also set FEC from the camera itself, which will affect the TOTAL flash exposure. Now, that's maximum flexibility!
What can be controlled: the Master flash can control each group's exposure and the ratio among the groups. It cannot control the units on/off status or the sequence of fire. They will all fire at the same time. The camera will control the TOTAL flash exposure which will include the master and any and all slaves in any group. That means that if you set Flash Exposure Compensation (FEC) in your camera ALL flash units are effected.
One easy setup is to use the Master (on-camera) flash as the main light with a diffuser on it and then, use the slave flash as a fill or accent light (with or without diffusion). Another easy one is to revers the role for the above setup: use the OFF-camera flash as the MAIN and the on-camera unit as a fill, to lighten up the shadows a bit (start with a setting of -2 stops for the on-camera flash and go from there, to taste). The photo below was done using the first setup suggestion.
This photo was done using a Canon Speedlite 550EX fitted with a Lumiquest Soft Box diffuser - the flash was mounted on a Canon EOS 10D and set as Master, controlling an off-camera 420EX, placed on a stand.
Another, very common setup would be to turn the Master flash head off so it will not fire and then, use only the slave with an umbrella/soft box as the main light, in a typical portrait configuration.
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