Background: How Does Autoflash Work? Automatic flash found on most modern camera systems works in one of three ways:

• Thyristor: as the flash fires, an external thyristor (typically mounted on a hotshoe flash) measures the flash light reflected from the scene. When the proper amount is reached, it cuts off the flash. This system is very cheap but not very accurate or reliable. It can give good flash exposure only for certain subject arrangements and distances.
• TTL or A-TTL: the through-the-lens (TTL) system has been used for three decades. As the flash fires, one or more sensors inside the camera body measure the flash light reflected off the film. This is more accurate than an external thyristor because the measurement accounts for the field of view and the light transmission of the lens and any filters. Ideally, the light sensors distinguish the illumination on the subject and avoid overexposure, though this is not always the case. Canon's A-TTL is an extension of TTL that links the flash and aperture settings for better results.
• E-TTL is a proprietary Canon system that appeared in the 1990s. Before the exposure starts, the flash emits a low-power "pre-flash". Because the mirror is still down, the camera's multi-zone exposure system can meter the light in many distinct areas throughout the frame. By comparing this to the original meter results without the flash (ambient light), the camera can determine what effect the flash has on any part of the frame. The selected focus point or other methods can identify the likely subject and adjust the flash power to properly illuminate that zone. Finally, the reflex mirror flips up, the shutter opens, and the exposure is made with a pre-determined flash power. Both the pre-flash and the main flash happen so quickly in succession that most people only see one flash.

Canon's E-TTL is very capable and gives consistently excellent automatic exposure in most circumstances. The system is found in all of the "EX" series flashes, such as the top-of-the-line 550-EX. It is also included with the built-in pop-up flash of newer consumer SLR bodies such as the EOS D60, 10D and 300D Digital Rebel. The E-TTL system also works perfectly with Canon's wireless master/slave system for the 550-EX, using infrared light pulses to command the 550-EX slaves. Some older Canon bodies do not support E-TTL and will not have the problem described below.

The E-TTL Problem with Optical Slaves

The idea of an optical slave is that the slave flash is triggered when it detects the light burst from the master flash. The slave contains a photodetector for this purpose. This works extremely well with slower shutter speeds (e.g., 1/60 second), so there's time for the slaves to respond with their own flashes. Because no sync cords are required to connect the optical slave, setup is faster and easier and there's less to trip over. And battery powered slaves can be placed in discrete locations in a scene without telltale wires.

The problem with E-TTL is that the slaves detect the pre-flash and fire right away -- before the camera's shutter is open. By the time the exposure starts and the main flash fires, the slaves have expended their charge and cannot fire.

Not only are the slaves then useless during exposure, they actually fool the camera into miscalculating the E-TTL flash power. The light measured during the pre-flash includes all of the slaves, so the camera concludes that less power is needed from the master flash. As a result, the exposure is made with too-little master flash power and zero slave power. It is futile to attempt flash photography with this configuration.

Note that a similar problem exists with the red-eye suppression feature of some cameras, in which a series of pre-flashes are fired. However, that feature is easily defeated -- and should be for any work with optical slaves.

Solution using Canon EX Series Flashes

To avoid the pre-flash (cancel E-TTL mode), there are two changes you can make to your Canon EX flash. One is to set the flash to Manual mode, where you determine the flash power (e.g., 1/8 power). The other is to program custom function 3 to a "1" on the 550-EX, which selects TTL mode rather than E-TTL. This appears to fool the camera body into thinking you've mounted a TTL hotshoe flash rather than an EX series. You can confirm the TTL setting in the regular LCD display. Either of these solutions will result in a single main flash for each exposure, but the latter retains autoflash (though not as accurate as E-TTL). Chances are, if you're using optical slave flashes, you don't need or want autoflash anyway.

Unfortunately, if you wish to use the Canon infrared flash remote system (ST-2E hotshoe transmitter or 550-EX in Master mode), these solutions will not work. Both will emit an infrared burst to communicate with the slaves before the exposure, even if the slave 550-EX units are all in Manual mode. This infrared burst will usually false-trigger any optical slaves.

Solution with Third Party Hotshoe Flash

Here's an easy, low-cost solution that doesn't require programming manual modes or custom functions. Buy a third-party hotshoe flash and use it to trigger the optical slaves. In most cases, the slaves provide the main lighting and so you don't need a high-power hotshoe flash (which means you'll get hundreds of flashes on a set of batteries). I do recommend getting a flash with a tilt head so that you can aim it away from the scene (toward the ceiling). Such a flash is available for less than $40. A more advanced flash for somewhat more will allow you to reduce the hotshoe flash power to the minimum necessary in order to consistently trigger the slaves. Note that the hotshoe flash does not need any autoflash functions (like thyristor), flash confirmation light, or anything like that.

Of course, you could purchase an EZ-series flash which gives you the option of lots of power, more features and compatibility with Canon bodies. I've use the 540-EZ and it is quite a nice unit. While you're unlikely to need TTL or A-TTL capabilities with multi-slave flash setup, the autoflash might be useful at other times.

One final note: I have heard that some studio flash units have been designed to ignore the Canon E-TTL pre-flash. I don't have any further information about what brands and models have this option, but it would be an easy feature to add. When selected, the optical slave would simply wait for the second in a burst of two flashes.