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 very coarse and inconsistent - easily foiled by subject arrangements and changing 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 flash system for SLRs. It appeared in the 1990s (Nikon's version is iTTL), and has since been refined (as well as reverse engineered by some compatible flash manufacturers). Before the mirror flips up to start exposure, 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. Ideally it correctly determines the subject (e.g., with selected focus point) and calculates correct flash power to properly expose that - even if just a little fill is needed. This all occurs in less than the blink of an eye. Now ready with its flash pre-metered, the camera flips up the reflex mirror, the shutter opens fully beginning the exposure, and the flash fires at the preset power. Both the pre-flash and the main flash happen so quickly in succession that most people only see one flash.

Of course, many people do not take the flash capabilities into such detailed consideration when making their camera purchases. Indeed, with so many great Sim card offers available, many people are happy to use the camera found on their mobile phone - many of which do not even have a flash. However, for those looking to take higher quality photos, these are the most common types of Autoflash.

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 580-EX. It is also included with the built-in pop-up flash of newer prosumer SLR bodies. The E-TTL system also works perfectly with Canon's wireless master/slave system, using infrared light pulses (line of sight) from a hotshoe transmitter to command the EX slaves (note that a master transmitter is included in the EOS-7D camera).

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. (Slaves, especially powerful ones, tend to be long-duration emitters, so slow shutter speed is necessary for correct exposure anyway.) 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.

Typically the slave flash power is set manually, either by previewing test shots and making adjustments, or using a flash meter during test shots. So we're not trying to use E-TTL in these circumstances. And, in fact, E-TTL can ruin the setup.

The problem with E-TTL is that the slaves detect the pre-flash (not the main flash yet). So the slaves 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 are recycling - they cannot fire.

Note that a similar problem exists with the red-eye suppression feature of some cameras, in which a series of pre-flashes are fired to undilate the subjects' pupils. However, the anit-red-eye feature is usually easy to turn off, and you should do that for any work with optical slaves.

Solution using Canon EX Series Flashes

To avoid the pre-flash, the easiest approach is to set the flash to Manual mode which uses a fixed flash power and does not use any E-TTL pre-flash. Adjust the on-camera flash to mix as you desire with the slaves. Or, if you don't want a lot of on-camera flash in the shot (because it looks flat without shadows), turn the on-camera flash power down as far as you can while still triggering the slaves (e.g., 1/32 power). This will also extend the service of your hotshoe flash batteries, since the flash hardly fires. You can also tilt and pivot the on-camera flash so it aims somewhere other than your subject, but casts light where the optical slave detectors can still sense the flash.

Another approach for Canon flashes is to program a flash custom function to use TTL mode rather than E-TTL. On the EX-550 this is CF 3, set to "1". You can confirm the TTL setting in the regular LCD display on the flash, where "TTL" will be displayed instead of "E-TTL". Apparently this reports to the camera body that you've mounted a flash without E-TTL capabilities, so the camera computer doesn't try metering with a pre-flash. While this retains some flash autoexposure (via TTL), I prefer to set everything manually. And when working without slaves, I don't want to fiddle around with flash custom functions to restore E-TTL capability.

Unfortunately, if you wish to mix in some Canon EX flashes with your optical slaves, you cannot use the Canon infrared remote system (ST-2E hotshoe transmitter or EX flash in Master mode). Apparently the transmitter's infrared emission (to communicate with the slaves before the exposure) will false-trigger some optical slaves. The emission occurs even if all of the slave EX flashes are all in Manual mode. If anyone knows a way to defeat this, please let me know. I'd like to use the EX flashes with optical slaves, but short of buying third-party optical triggers (or radio links) for the flashes I don't think it can be done.

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) when your lighting design does not call for direct front flash. Simple hotshoe flashes (without user adjustments) are available for well under $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, keeping a clean lighting design.

One final note: some third-party optical slave flashes have been designed to ignore the Canon E-TTL pre-flash, as well as the pre-flash from other camera manufacturers. For example, I have a very-inexpensive underwater optical slave that can be set to count various numbers of pre-flash pulses before arming for the "real" flash.