Canon EOS Cameras: TTL Flash Metering Types

TTL (Through The Lens): Off the film metering (OTF)

A-TTL (Advanced TTL): Uses a preflash to determine the required amount of flash light by use of a sensor in front of the flash, but the duration of lighting is still determined by an OTF sensor during the shot.

E-TTL (Evaluative TTL): Uses a preflash to determine the required amount of flash light by the use of the camera's own exposure metering system. See here for more details on flash basics.

E-TTL II: Even more evaluating and communication between flash and lens to the body, with distance data transmission being the most commonly referred additional factor. That said, to use E-TTL II it is mandatory to use a lens that transmits distance data.

Common sense is, that E-TTL II is a camera feature and does not need special flashgun support beyond E-TTL in general. But Metz indeed testified, that this specification does rely on flash support for transmitting zoom reflector position and corresponding guide numbers as well as color temperature. Furthermore analog and digital E-TTL differs in protocol and at least by duration and amplitude of the preflash (which is given as reason for the Metz MB 40-MZ3i not to work with D-SLRs). Considering all of this leads to the assumption, that all these preferences might change from body to body, even within E-TTL II D-SLRs, and thus for example Cullmann 34 AF/C Digital will work with EOS 20D (E-TTL II 2004) but not EOS 5D (E-TTL II 2005).

X-Contact: Non-TTL full power flashing unless flash features manual control or is computer controlled.

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