>From the Canon website (Europe) these are the best explanations of A-TTL and
E-TTL I have seen.
Evaluative Through-The-Lens Mode (E-TTL)
E-TTL (Prestored Evaluative Through-the-Lens flash exposure control) is a
new flash mode that reads through the lens, but not off the film. Utilizing
a preflash fired after the shutter button has been fully depressed but
before the camera's reflex mirror goes up, E-TTL uses the camera's
evaluative metering sensor to analyze and compare ambient light exposure
values with illumination reflected from the subject by the preflash. This
data is used to calculate and store the flash output required for optimum
exposure of the main subject (identified by the active focusing point),
while maintaining a subtle balance between foreground and background. Though
transparent to the user, E-TTL represents Canon's most advanced flash
exposure control system to date.
Advanced Through-the-Lens Mode (A-TTL)
The predecessor to E-TTL, A-TTL (Advanced Through-the-Lens) is used with
Speedlite 540EZ when set for direct flash in the camera's program mode. (TTL
flash exposure control is available at other times.) Like E-TTL, A-TTL reads
through the lens and concentrates its sensitivity on the area of the picture
covered by the active focusing point. Unlike E-TTL, A-TTL controls flash
exposure with a dedicated sensor that reads flash illumination reflected
from the surface of the film during exposure. A-TTL also uses a preflash,
but unlike E-TTL, the preflash is used to calculate an aperture value based
on the distance that light must travel from the flash to the subject. The
camera compares the distance-based aperture data to the ambient-based
aperture data calculated by the camera's metering system, and selects the
smaller aperture value of the two. This method ensures accurate exposure of
the subject in any lighting condition. Speedlites equipped with A-TTL are
fully compatible with all analogue EOS cameras. EOS D30 uses only E-TTL
speedlites.
For more check out
http://www.canon-europa.com/products/accessories/speedlites/tech/t_speedlite
_tech.html
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