7.3. A-TTL flash metering

"Advanced" TTL flash metering is a Canon TTL variant available to all TTL-compatible film cameras when a Speedlite EZ model is attached.

In A-TTL mode, the flash unit fires a brief pulse of light during the metering phase, before the shutter actually opens. A-TTL units have a special sensor to record light bouncing back from this preflash, and that data is factored into normal TTL operation. In other words, the A-TTL preflash is supplementary, does not supply the primary metering data, and is measured by the flash unit and not the camera.