You're partially correct - a CdS cell cannot react to light from a
strobe. Even changing the intensity of light two or three "stops" will
take several seconds to reach full impact on a CdS cell - and the
typical maximum duration of a strobe is in the neighborhood of
1/10,000th second, often faster.
One correction on the XA's function, however - they actually use two CdS
cells, one of which controls only the meter needle in the finder, the
other being connected to the circuit that controlls the exposure. There
is no "memory" function - in other words, the exposure isn't "set" when
the release button is pressed, it is judged in real time. The exposure
control circuit keeps the shutter's electro-magnet energized (keeping
the shutter blades opening/open) until it registers that enough light
has struck the CdS cell to provide a proper exposure - then it
de-energizes the electro-magnet, allowing the blades to begin closing.
The exposure during this closing time is included in the exposure
calculation.
rdk@xxxxxxxxx wrote:
>
> I was of the opinion that, for fill-in flash with an XA, one used the flash at
> full power, where the sensor is not used, so no flash quenching takes place.
> The
> camera sensor is a cds cell, not a Silicon Blue Cell (SBC), so I do not think
> that it could react fast enough to measure light during exposure; the shutter
> speed is determined at the moment you press the shutter release, not during
> the
> exposure.
> I think you have to use the flash GN (11 for A11, 16 for A16 at 100asa) to
> determine the aperture, which should be about half that indicated by a normal
> GN
> flash calculation for the required distance (i.e f11 if the calculation gives
> f8).
>
> regards
> Roger Key
>
> >I would think that it would work as follows:
> >Turn on flash
> >Set appropriate aperture for the flash you are using.
> >Trip shutter.
> >Flash fires.
> >Flash quenches when enough light has reflected back to flash sensor.
> >Shutter closes because camera sensor has just seen enough light (from flash)
> >to
> terminate the exposure.
> >So even if the ambient light required an exposure of 5 sec., the exposure
> actually ends when the flash is quenched. >(not considering various
> electonic &
> mechanical delays inherent in the system).
> >What do you think ?
> >...Wayne
>
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