P3D Re: flash synch

[boris@xxxxxxxxxxxx (Boris Starosta)]

>
> Recently I've been wondering how to synchronize flash with a twin 35mm
> set-up - cheap. I thought about a salvaged Radio Controll unit, servo,
> solenoid. Since I know nothing about this ; it would be a  time

David:

There's more to synchronizing cameras for flash use than meets the eye
behind the film gates, as I learned the hard way.  Synchronization appeared
easy for me, as I was lucky enough to find a high quality adjustable
mechanical twin release.  Synch was apparently so good, that I could freeze
water droplets in midair, and they would image correctly in stereo!  The
problem is that while both shutters may appear open pretty much at the same
time, that is not good enough for flash photography.

Much experimentation and many rolls of film still have not resulted in a
satisfactory solution.  So I join with David in a call to P3D for a
possible solution.

Here's what I have learned so far.  First of all, you can only trigger the
flash off of one of the cameras.  With most cameras, the flash is set off
just as the camera's shutter reaches all the way open (When the opening
curtain completes its trip across the frame).  Logic requires that this
camera's shutter is lagging slightly behind that of the non-flash camera.
You want the non-flash camera's shutter to be all the way open, before the
shutter of the flashing camera is all the way open, because that is when
the flash will go off.  If the flashing camera opens first, even by a
slight fraction of the exposure, the other camera will record only part of
the frame, because the flash will illuminate a still partially closed
opening curtain.

You will rarely see a good set of exposures, if the cameras are set at the
fastest synch speed (1/60 for my Pentax). That's because at that setting
the shutter is fully opened for only an instant, then begins to close
again.  That is the fastest synch speed by definition.  At higher speeds
the shutter is never all the way open at any one time.  At 1/500, the
closing curtain is following the opening curtain fairly closely across the
frame, so that a flash set off during such an exposure would illuminate
only a fraction of the frame.

What I found with my Pentax pair of cameras is that even at slower synch
speeds (as low as 1/15), where the two shutters would surely be open long
enough to both capture the flash reliably, I was still losing a significant
number of frames on the non-flash camera to shutter problems.  The amazing
thing was that I was still getting errors on both sides of the synch:  many
partial "premature closures", some partial "not yet opens".  I surmized
that the Pentax mechanism simply has too much slop and variability, and
that my earlier water droplet pictures were just dumb luck.

My solution so far has been to shoot one half second exposures, and trigger
the flash manually during that half second.  Obviously, I can only do this
in a fairly darkened studio.

I would appreciate any suggestions, also.

Boris



Boris Starosta

usa 804 979 3930

boris@xxxxxxxxxxxx
http://www.starosta.com
http://www.starosta.com/3dshowcase

"A lifetime can be well spent correcting and improving
one's own faults without bothering about others."
- Edward Weston

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