Dear Ray,
In other words, set the flash computer for your film ISO, note the
f/stop suggested, and set your camera to one stop narrower for 1/2
fill. e.g., flash says f/8. Set camera at f/11 for 2:1 ambient:fill,
f/16 for 4:1, etc. Set (or watch on auto) shutter speed to give
proper ambient exposure, but equal to or longer than 1/60th for
proper sync.
Most flashes give you several f/stop settings for a given ISO.
Tom
On 7 Jan 2001, at 18:33, sayeth Ray Moth <olympus@xxxxxxxxxxxxxxx>
> 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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