Hi,
I wonder:
If the A-TTL sensor reads from the surface of the film, how could it read
the pre flash? Moreover, how come the pre flash is fired only when the flash
is directed towards the subject to calculate the distance, when this
information can be fed by the lens (sort of Nikon's D-TTL)?
Luke
-----Original Message-----
From: Henry Posner/B&H Photo-Video
Sent: 25 Januari 2001 0:13
To: [EMAIL PROTECTED]
Subject: EOS Re: A-TTL and E-TTL
I hope that <one paragraph> was just a suggestion. :-)
This was given to me by Canon's Chuck Westfall. It was written a year or so
ago and does not mention some newer products.
Advanced Through-the-Lens Mode (A-TTL)
======================================
.... 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 normal metering system, and selects the smaller aperture value
of the two...................
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