In article , Lars Haven <lhaven@xxxxxxxxxxxxx> wrote
>Kennedy wrote:
>>
>> >
>> >...because it only happens at about 1/10000th, doesn't it, hardly a
>> >problem for most brands of camera, let alone Olympus.
>> >
>> But what about brands of flashguns - one of which is Olympus? ;-)
>>
>> The minimum duration of the T32 is 1/40,000th and I seem to recall some
>> of the bigger Metz flashes go down to 1/250,000th.
>
>You would expect the ultra-fast exposure of an electronic flash
>at short range to cause some reciprocity failure. That it does not,
>is probably best explained by a slight electronic delay and afterglow
>in the flash tube. This will cause the flash to overexpose a little,
>especially when making very short flashes, and thereby compensate
>somewhat for the reciprocity failure.
>
1/250,000th, or 4uS, is an age in electronics - your PC could probably
calculate pi to quite a few places in that time if programmed
appropriately, so I don't buy that one.
Afterglow? Its a long time since I did any plasma physics, but I think
the time for the spark to decay once the energy source has been removed
(via the thyristor in the flash circuit) is of the order of pico-
seconds.
My guess is that the times at which reciprocity starts to reappear are
closer to 1uS mark or less, rather than any failing of the circuit to
control the flash fast enough. Of course reciprocity failure might just
have a slower fall off at the high speed end than it has at the long
time period - the mechanisms are probably completely different - so its
a bit less noticeable.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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