Dirk Wright wrote:
I'm thinking hard about this before a full cup of coffee, so maybe this
is wrong, but if the film speed setting *does not* effect flash output,
then how does the flash know what speed film is being used in the
camera? For the flash to determine proper exposure, it needs: film
speed and reflected light from the thyristor, correct?
=====================================================================
As far as I know, a flashgun has, typically, three selectable levels of
output ('doses') in auto mode. The intensity of each of these 'dosage'
levels is controlled by the flashgun and depends on the light reflected
back from the subject, which is measured by the flashgun's sensor. The
illumination of the subject is controlled to allow for variations in
flash distance and subject reflectivity.
The illumination of the subject does *not* change with the speed of
film being used; what does change, however, is the lens aperture that
would be appropriate for each flash 'dosage' level, to achieve correct
exposure. This obviously depends on the film speed. E.g. a particular
auto setting on the flashgun ('dosage' level) might require the lens to
be set at f/4 for ISO 100; for faster films, a smaller aperture would
be required: f/5.6 for ISO 200, f/8 for ISO 400, and so on. The
availability of three 'dosage' levels is provided so that there is some
flexibility in the aperture settings that can be used.
There is usually a built-in sliding scale on the back of the flashgun
that can be set to the required film speed and shows the aperture
should be set for each of the 'dosage' levels. The setting of this
scale does nothing to the flashgun, it is merely a user guide.
Regards,
=====
Ray
"The trouble with resisting temptation is
you never know when you'll get another chance!"
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