Slave flashes are simply self-contained flash units which respond to external triggers of some kind. They're frequently used in studio situations. For example, you might have a multiple-flash setup - one flash to illuminate the subject and another unit or two to illuminate the background separately.
Many slave flashes are triggered by light: optical slaves. They have small sensors built in or attached that detect the light pulse from another flash unit and then trigger immediately themselves. Since they respond so rapidly, the time delay between the trigger flash and the slave flashes going off does not affect the exposure of the photo. The Wein Peanut, a tiny and inexpensive accessory, is a popular optical trigger that's basically compatible with most flash units out there. (though ironically not entirely compatible with a lot of Canon Speedlites - see further down in this section for details)
Since the sensors watch for flash bursts you thus use one flash unit as the triggering flash - typically the built-in flash unit on your camera or an external unit connected to the camera's hotshoe or PC connector. The triggering flash can be set to a low power output so that it doesn't affect the scene if you want - optical slaves are usually sensitive enough. The slaves are also usually sensitive to infrared energy, so another popular trick is to tape an infrared filter gel over the internal flash unit. This lets you trigger the flash units with a burst of light* that's invisible to the human eye and to most types of film. (* okay - to be pedantic, IR energy is not light since the human eye can't see it, but "infrared light" is a useful concept if technically inaccurate)
Canon E-TTL flash metering poses a problem for this sort of setup, since standard analogue optical slaves are likely to be triggered by the preflash rather than the actual flash. Standard optical slaves are also a problem outside the controlled environment of the studio. They're a real nuisance in wedding photography when, for instance, Uncle Charlie's point and shoot camera flash triggers your optical slaves. Situations like that call for expensive radio-controlled wireless systems or, if battery-powered slaves have enough power output for your needs, Canon's E-TTL wireless system. An alternative is the new generation of optical slaves, such as the Wein Digital Smart Slave products, which are capable of distinguishing between a preflash and a genuine scene-illuminating flash and only respond to the latter.
A significant problem with multiple slave flash photography (at least, one which doesn't rely on automated metering like Canon's wireless E-TTL) is that it's difficult to preview or visualize the final result without a lot of testing and experience. Usually each flash unit has to have its output set manually. In fact, unless you're replicating a predetermined lighting formula that works for you or you're configuring a fairly simple one or two flash setup with a light meter, I'd say that it's pretty well a requirement that you have a Polaroid back for your film camera or a digital camera to do this sort of thing well. Digital is particularly beneficial since you can take dozens of test photos at no cost and determine exactly how the various flash units are illuminating your scene, where the shadows fall, etc.
However, it can be an affordable way to set up your own studio. Buy a few old battery-powered Vivitar 283s or second-hand studio units, slap some cheap optical triggers onto them and you're in business.
Canon do not build any flash units specifically intended for use as studio equipment. However, you can buy hotshoe adapters - optical or wired - to turn any flash you want into a slave, and the 480EG can be slaved via the optional Synchro Cord 480. Hotshoe adapters aren't always reliable with every camera and flash unit combination, so it's worthwhile doing some testing first. In particular, a lot of people have reported problems with small optical slaves not being able to trigger Canon Speedlite flash units more than once without the flash being turned off and turned on again between each shot. The Ikelite Lite-Link is one device designed to work with Canon flash units that apparently does not have this problem. It also has a sort of simulated TTL feature - it can cut the light from the slave flash as soon as the master flash has quenched its light, rather than simply firing at full power.
Finally, Canon state in their literature that a sync speed of perhaps 1/60 or 1/125 is required for studio flash. There are two reasons why they suggest speeds this low, even if the camera's capable of higher flash sync with TTL-metering portable Speedlite flash units. First, many older studio units take quite a while to attain full brightness or have slight colour shifts depending on the flash duration. And second, the triggering delay (the time that elapses between the camera triggering the flash and the flash unit actually firing) with slaved studio flash units is often longer than the very brief and known triggering time with TTL flash units.
For these reasons you're probably best off doing a series of tests with a new slaved flash unit setup at different shutter speeds to determine what the top shutter speed for your configuration is going to be. Particularly with optical and radio slave units or older flash units.
Note that Canon do sell a number of flash units that can serve as slave units in a wireless E-TTL setup - see the section on wireless E-TTL for details.
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