On Thu, 25 Jan 2001, Lukas Lauw wrote:
> 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
The preflash is read with a sensor in 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)?
I think one problem was that not all the lenses report the distance. And
what is the accuracy of the reported distance anyways? If it's as accurate
as the distance scales on lenses, then... well you know... of course it
wouls be done by electronics, and thus the visual scale doesn't give much
clues - but how about the thermal expansion? That's why the Canon lenses
can be focused beyond infinity - infinity isn't at the same place always
:-) So that just makes me wonder how accurate that info from the lenses
is. Does 1V record also the distance?? If so, maybe someone could give use
some insight to the distance info...
> -----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...................
BTW, this selecting the smaller aperture of the two, is the thing that is
wrong IMHO... makes the background dark very easily and doesn't even try
to balance the ambient light with the flash... This is why I think that
plain TTL is better - I'm in control :-)
Best regards,
Hugo.
************************************************************
** Hugo Gävert **
** [EMAIL PROTECTED] http://www.hut.fi/~hugo **
************************************************************
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
