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Making Multiple Flashes Work Together

The Canon E-TTL flash system allows the use of multiple flashes to achieve lighting effects not possible with a single flash. The 550EX flash unit and the ST-E2 flash controller have the ability to control an unlimited number of flash units when attached to a Canon camera body. The unit attached to the camera is referred to as the master, and the flashes controlled by the master are called slaves. The 550EX flash can be a master or a slave, and the 420EX flash can only be a slave. Communication between the master and the slave(s) is accomplished by pulsing the flash tube of the master in a sort of ultra-high-speed Morse code that broadcasts the desired power level and other information from the master to all slaves within range. Canon doesn't actually use Morse code, but the concept is similar.

E-TTL has 4 separate channels so that multiple photographers can work in the same area without firing each others' slaves. Each channel has 3 groups; A, B, and C. All flashes in the same group will fire at the same power level, but each group can fire at a different power level. As mentioned earlier E-TTL fires a metering flash before the actual exposure to determine the correct power level for the exposure flash. When the flash attached to the camera is set to be a master, E-TTL will actually fire 3 metering flashes; one for each flash group. The correct power level is calculated for each flash group independently; when the shutter opens, all the flashes in each group will fire at the power level calculated for that group. The master flash is always assigned to group A, but slave flashes can be assigned to any group: A, B, or C. The amazing thing about this is that it happens so fast that there is no additional shutter lag when compared to shooting without flash. Canon 1 series bodies have a 55 millisecond delay between the shutter release being pressed and the shutter opening, whether shooting with multiple flashes or without any flash at all.

Using a 550EX as the master flash allows for more flexibility than using the ST-E2. The ST-E2 can only control groups A and B; the 550EX can control groups A, B, and C.  With the 550EX, your ratio options are Off, A:B, and A:B:C. If you select A:B, flashes in group C will not fire. The ST-E2 cannot be used as a flash, only a flash controller. When using flash ratios between groups, the overall power level is determined by the camera and the Flash Exposure Compensation setting, and then the power is split up between the groups according to the ratio you have selected. Groups A and B are set as a ratio to each other, A:B. Group C is set as an amount above or below the overall power setting, +/- 3 stops in 1/3-stop increments.

Practical Application:

Triple Flash Setup using E-TTL.


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