In article <32E28E91.483A@iamerica.net>, melbrown@iamerica.net says...
>David N. Heller wrote:
>> Curtis Leeds wrote:
>> > David N. Heller wrote:

>> > > You are correct. Both TTL and sensors on flashes measure light
>> > > reflected from subject from all sources, not just the flash.

>> > Sorry, but this is totally false; flash sensors only detect intermittent
>> > light. This is why your flash will fire - and produce proper fill-flash
>> > exposure on automatic - even in bright direct sunlight.

>> Curtis, you are correct. Thank you for alterting me to my mistake.

>David, I say you were correct the first time. It is true that the sensor
>of a flash meter, or a TTL sensor in a camera, triggers only on a sudden
>increase of light level (as provided by the flash), but when it does trigger,
>there is no way it can separate the flash and ambient light intensities; it
>measures them together, as you first said.

Hmmm, in a way, you are all technically correct, depending on how you
look at it... During the measurement period, all light from whatever
sources is measured (and if the electronics are there [in the camera]
to adjust the flash light relative to the full ambient metering, flash
fill, rather than full flash, can be provided), BUT, the ambient light
appears in the metered mix as an equivalent shutter-speed/aperture
combination, and therefore will probably have little influence on the
outcome unless the camera shutter speed is high enough to approach the
speed of the flash exposure. A way of looking at it is this example:
the ambient-light exposure is 1/125th at f8; the TTL flash duration
for the same conditions may easily be 1/2000th (or shorter); the
ambient-light contribution to the flash metering period of 1/2000th
second in this example would be under exposed by four stops, and
therefore unimportant. Shooting a leaf-shutter camera at 1/500th
with a flash that is used near full power (dropping its exposure
time to near 1/500th second) does present problems with accurate
flash metering, but maybe also provides a sort of auto fill-ratio
as a result...).
Hope This Helps