Cameras with built-in wireless flash control use the pop-up flash to emit visible light flashes to "communicate" with the remote flash. The distance they can reach will vary depending upon ambient light and is line-of-sight only. If used continuously for this purpose, after awhile the pop-up flash will shut down to prevent overheating. It will be usable again after it's had some time to cool down.
Cameras without built-in wireless flash can be fitted with one of the larger flashes that can act as a "master" and control off-camera flashes.... Or it can be fitted with an ST-E2 flash controller module. Both these use near IR light to communicate, largely invisible to the eye. It is a bit limited in distance and line-of-sight only, too. These are not subject to the same overheating prevention shut downs as the pop-up flash (though any flash will shut down temporarily too, if worked too hard).
There are now also 600EX-RT flashes and ST-E3-RT, which are radio triggered off camera flash and controller modules. These have the greatest reach and are not limited to line-of-sight only.
There are also third party optical and radio controllers that are merely "dumb" triggers... firing the flash but not really communicating with it or controlling it in any additional way. These can be rather inexpensive, but your flash setup and exposures are manually controlled.
There are a couple third party radio controllers that do offer relatively full off-camera control of the IR-based flashes. These are fairly expensive, but offer TTL flash metering and multi-flash control
Incidentally, the flashes and ST-E2 that communicate in near IR, also can emit a Focus Assist beam that's near-IR and a whole lot less intrusive than the rapid strobe white light of the pop-up flash serving the same purpose. (The ST-E3-RT does not offer a Focus Assist feature).
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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