At 11:52 PM -0400 6/24/02, [EMAIL PROTECTED] wrote:
>Doesn't the older A-TTL mode fire a
>pre-flash to gauge the distance, not the reflectivity? But any time you use
>Av or TV for auto-fill flash or a bounce angle on a 540EZ, it goes from
>A-TTL to plain old TTL, which measures reflectivity.
Well, depends what you mean by reflectivity. TTL flash metering
relies on measuring flash-originating light reflected off the surface
of the film. A-TTL flash metering does indeed include an infrared or
visible light (depending on the model) preflash to determine
distance, but it does so by measuring reflectivity back from the
subject.
The information returned by the A-TTL preflash is kind of useless,
however, since it only appears to be used in P mode to stop down the
aperture and (with EZ flash units other than the 540EZ) in the other
metering modes to set the out of range light on 600 series EOS
cameras.
>I wish Canon would just document everything so that we know what to expect,
>or make it consistent or give us a way to cancel automated compensations.
I'd have to agree. Vague documentation of metering - flash and
ambient - is indeed a problem.
>[...] I remember an old rumor that Canon can't use
>distance info from lenses to calculate flash exposure with guide numbers
>because another maker (Nikon?) owns the patent on the idea and Nikon won't
>grant or Canon won't pay for a license to use the patent. [...]
Possibly. There's also the fact that distance calculations are only
of value when shooting with the flash pointing straight ahead and
with no diffuser. If you have a flash head diffuser or if you're
doing bounce flash then distance data is of no particular value.
- Neil K.
--
t e l a computer consulting + design * Vancouver, BC, Canada
web: http://teladesign.com/
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