Gary Fisher wrote:
>>
>> > A-TTL is useless because it's only available in
>> Program mode
>>
>> This is only true for the 540EZ, the older flashes
>> do A-TTL in AV and TV
>> as well.
>>
> Hi Thomas,
>
> It's true the 430/420EZ emit the preflash in Av and
> Tv, but I don't believe they actually use it for
> a-ttl. This behaviour has been discussed before on
> the list.
>
> If you're using Av or Tv modes, then the camera
> exposes for ambient lighting always and the flash is
> treated as fill flash. The flash exposure does not
> change the aperture setting or shutter speed in any
> way. The camera uses plain ttl for exposure.
Yes, that's A TTL, it's basically a fill flash mode. It tries to fill
the main subject, which is supposed to be found at the active AF-sensor.
>
> A-ttl is for the camera to try and calculate aperture
> and shutter speed (in Program mode). In Av and Tv it
> is not doing this - you are. The preflash that occurs
> is ignored in those modes.
>
This is right, kind of anyway. The preflash doesn't seem to make much
sense in AV and TV, because the aperture is either set by the user or
determined by the ambient exposure needed. With older cameras it was
used to give a flash out of range warning prior to the exposure.
Even with new cameras, what you get in those modes is not quite plain
TTL. When the flash is enabled, the camera changes to a special kind of
evaluative metering for ambient light. The zones around the active AF
sensor are ignored in this case. This is also somewhere in the flash
FAQ.
Thomas Bantel
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