Canon's E-TTL system makes complex flash exposure decisions on the fly, using complex algorithms.
Canon's E-TTL flash system, introduced as part of the EOS camera system, is a method of syncing flashes and evaluating the appropriate flash power on-the-fly. E-TTL stands for Evaluative Through The Lens, which means that the flash power is evaluated by a software algorithm which looks at the light coming through the lens, evaluates the scene and the focal points, and adjusts the flash to the appropriate power setting to give the correct exposure for the scene. Many flash units are compatible with this standard, including several third-party flashes.
Canon Flashes
Canon makes a large number of flashes that work with E-TTL metering. While an exhaustive list would be prohibitively long, common compatible units include the 270EX, 430EX, 430EX II, 550EX, 580EX and 580EX II. Several other discontinued flashes are also compatible with E-TTL metering, and any E-TTL II flash is compatible with E-TTL.
Metz Flashes
Metz has made a name for itself by producing third-party imitations of name brand flashes that are fully compatible with proprietary metering systems and selling them for less than the name brand's hardware. Metz also offers a wider range of flashes than Canon does, as well as some features that Canon flashes do not have, like memory settings and audio warnings. While the compatibility of these flashes is not guaranteed, Metz flashes perform better than third party flashes in this regard, but also require an adapter to work. Flashes include the 54MZ-3, 50MZ-5, 40MZ-3, 40MZ-1, 40MZ-3i, 40MZ-1i, 40MZ-2, 40AF-4 and 32MZ-3.
Sigma EF-500 DG Super
Sigma also produces a flash designed to imitate Canon's 550EX, and does so quite well. The flash has gained a reputation for working as well as the 550EX, and even has wireless E-TTL compatibility. While the construction quality is slightly flimsier, the EF-500 is about half the price, even if the interface is clumsier. The flash itself is less powerful and there are no custom functions available.
Sunpak AF5000
Sunpak is a well-regarded name in flashes from the days of film photography. They carry the AF5000, which is TTL only, meaning it does not take advantage of the more complex evaluative algorithms of Canon's E-TTL system. TTL does allow for automatic exposure computation, however, and the AF5000 is very inexpensive. If you're willing to shed features to save some money, this is the way to go.
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