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Re: [OM] FW: T32 is not firing at 1/60 but at 1/30 downwards

Subject: Re: [OM] FW: T32 is not firing at 1/60 but at 1/30 downwards
From: "Julian Davies" <julian_davies@xxxxxxxxxxxxxx>
Date: Wed, 10 Sep 2003 21:32:34 +0100
It is not possible to switch off the quench circuit on a T series flash.
Therefore in manual mode it will go to the selected power UNLESS it gets a
quench signal from the camera body.
The quench signal from an OM2(n) is generated directly from the coil on the
shutter closing curtain release magnet. This is connected directly to the
quench contact on shoe 2/3/4, and is not disconnected by switching to manual
mode.
Now for the timing.
The OM2 does NOT meter or measure the opening time of the shutter curtains.
It measures the light falling on the SBCs from the moment that the meter
start switch is triggered by the rising mirror. From then on it integrates
the light (auto) or the current (manual) until the comparator circuit says
enough and releases the magnet which is holding the closing curtain. There
is a short delay while the relevant parts move, and the curtain runs. By
adjustment of the position of the start switch, the delay between meter
start and first curtain release is adjusted to be the same as the delay
between magnet release and second curtain run.

Now think about a sync of 1/60s. At the limit, it means that the second
curtain starts to run at the instant that the first curtain completes its
run. this is the same instant as the X sync trigger is completed.
but the timing of the exposure started BEFORE, so the magnet will release
BEFORE also. Since this sends the quench signal to the camera, the flash
will quench before it is triggered!
Now make some allowances for tolerances etc. and realise that the limiting
instant above  is a definite period of time. You get to a situation where
the quench signal may be sent from a time before the trigger to a time just
after. Depending on your camera, this may make the flash appear not to fire,
or fire with a reduced GN.
This is the prime reason why OM2(n) is limited to 1/30s (manual). It has the
same shutter and X sync circuit as OM1, but OM1 doesn't send the quench. At
1/30 (manual), it is guaranteed that the quench signal is not sent until
AFTER the second curtain completes its run. Also, if the flash doesn't care
about the quench signal, 1/60 will work fine.
To see if this is your problem, tape over the quench contact on the shoe.
In auto, of course, the light measurement starts, the shutter opens, the
flash triggers, the flash illuminates the subject, the meter says enough and
the magnet drops. This quenches the flash, and the assumption is now that
the scene is in relative darkness, so the delay in actually closing the
shutter is irrelevant. Note that 1/30s is used in manual simply because it
is the next slowest selectable speed. with auto having no fixed speeds, the
range is more flexible, and extends most ofthe way up to 1/60, but not all
the way.

Julian



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