Chip,
Your point of confusion is that you think the camera is controlling the
exposure by controlling the closing of the shutter. The shutter speed
is constant at whatever the camera needs for synchronization. The
exposure is controlled by electronically limiting the duration of the
flash (quenching the fire, so to speak). So, to use a slave flash via
TTL control there must be a means of sending an electronic signal (fast)
to the flash to tell it to shut off. The speed must be small fractions
of a millisecond... something the shutter can't do.
That also explains why a normal flash doesn't work as a TTL flash. It
has no provision for an electronic input signal to tell it to shut off.
It simply dumps its entire charge and you allow for but don't control
the light level.
----------------------------
Chip wrote...
I'm confused. From previous notes, I'm under the impression that my
OM-4
(with a T-32 or T-20 on it) will also include a slaved flash unit in
it's
Off the Film Plane exposure control (see Ken's note below). In other
words,
the OM-4 measures the exposure needed - weighing all light coming
through to
the film plane; TTL sources and others - and terminates the shutter upon
proper exposure.
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