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Re: [OM] F280 Super FP revisited

Subject: Re: [OM] F280 Super FP revisited
From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
Date: Mon, 9 Sep 2002 00:46:25 -0700
At 22:48 9/8/02, Pete wrote:




  If true, and I believe it is, then the flash duration will be the
shutter speed plus the time before and after the shutter operates.
And, the longer the shutter opening, the less flash power that will
be available, according to the charts.

No.  With focal plane shutters, total time it takes for a curtain to
travel across the film gate remains constant, regardless of shutter
speed.  You have two curtains, an opening one (leading) and a
closing one (trailing).  What varies with shutter speed is the delay
between releasing the opening one and releasing the closing one.  In
other words, it's the *gap* between them that changes.  Shutter
speeds faster than the X-sync speed have a gap that is narrower than
the film gate.  At those speeds, the closing curtain is released
before the opening one has completely traveled and the result is a
slit moving across the film gate.  The faster the shutter speed, the
narrower the slit, but the slit itself *always* moves at the same
speed.  Therefore, for an "FP" flash to work, it must always have
the same, long burn duration, regardless of shutter speed.  And . .
. the narrower the slit traveling across the film gate, the less
light from the flash that's allowed to reach the film.

-snip

-- John


John,

If a speed higher than X-sync is set it also means that the second
curtain is released sooner rather than waiting for the 1st curtain to
complete its travel.  The second curtain closes sooner, not because
it moves any faster, but because it was released sooner.  The FP
flash is quenched sooner as a result. That is why Olympus can give a
Super FP Flash mode specification of 80 to 260 firings on a set of
batteries. The specs also give a Super FP flash duration of 1/50 sec.
to 1/25 sec. with the OM4T/Ti. Also given is recycle time with AA
batteries, "Super FP Flash mode: 5-10 seconds depending on shutter
speed."

The important thing to remember is the Super FP mode, though not very
bright,  is a constant intensity and longer duration than the rated
shutter speed.  The relationship between its output and the ambient
light is determined by the ambient light level and the distance to
the subject.  FP mode is sort of like one of those P&S tiny flashes
that are only good at normal flash distances using ISO 400 film. Lots
of power in TTL mode though.
--
Winsor Crosby
Long Beach, California

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