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filmscanners: Canon's flash metering

This is OT posting, just hope the List moderator will excuse me this time. I
just failed to figure that out searching for the info in the Net.
I just read about Canon's flash exposure systems which referred to two major
types: A-TTL and E-TTL.
If there are also current or former Canon users on our List (or just from
their general knowledge) I would be glad to hear their opinions.

As far as I understand, both A-TTL and E-TTL use pre-flash to determine
subject-related information to fine tune their flash exposure during the
main flash burst. However they use pre-flash measurement information in
different ways (A-TTL still uses OTF main flash measurements while the E-TTL
process it in some other way).

First of all, I'm still confused how exactly both work, what is exact
differences between them, which algorithm is better for particular
applications and how by far they are distinguished from Minolta's regular
center-weighted TTL flash metering. Any ideas ?

I've heard A-TTL flash measurements are linked to the active AF sensor
for given exposure, so can handle well off-center subjects (of course if
focused properly). Is that correct ?

Alex Z


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