> Matthias,
>
>> The trick with pre-flashes and stopped down irises is, that because of the
>> stopped down iris less light reaches the red retina. So the inner part of
>> the eye keeps dark enough not to be detected by the film.
>>
>>
> How long does the iris take to stop down ?
>
> What are the dependencies ?
>
> ...Wayne
I dont know, how long the iris take to stop down. Under stress, if you
observe your iris in a mirror and imagine something really stressing (for
example the speech you have to hold tomorow), the reaction to this
imagination can happen almost instantly (less than 1/4 second). The
reaction time with bright light may be slower, but I think it should be
shorter than the time lag between the pre-flashes and the flash of cameras,
which do this trick. One dependency I can think of is, that people who have
drunken alcohol maybe have a slower or no reaction to pre-flashes. This may
be the case sometimes with party photos. Further I have read independent
tests which say, that cameras do this trick with different results, the
Olympus technic was mostly among the best.
I wanted to add to the above explanation, that you can imagine the inner
eye with closed iris as underexposed due to the small "aperture" in
comparison to the whole motif.
Matthias
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