[photo-3d] Flash sync speed, was Re: externally twinnables & flash

[boris@xxxxxxxxxxxx]

Jim Norman wrote in digest 373:
...
>of both cameras.  As a general rule, I try to use shutter speeds at 1/30 sec
>or slower when I'm using flash.  I can't explain why, but that increases my
>success rate dramatically.  Even so, every once in a while, the two cameras
>will not fire at the same instant, creatingt a dark band across the image
>that was produced by the camera that didn't have the flash mounted on it.

As I understand it, in focal plane shutter cameras flash sync speed is the
fastest speed at which the shutter of a camera is fully open for at least
an INSTANT - where the leading (closing) edge of the 2nd curtain does not
start its journey across the film gate until the trailing edge of the first
curtain has gone all the way across.

This is a function of how fast those curtains can get across the gate.  If
you have lighter shutter curtains (like titanium), then they can be moved
faster with a given amount of push (or pull.  Momemtum = mass x velocity.).
And this allows a faster flash sync speed.  If one curtain can get across
in 1/250 of a second, then that is your fastest sync.  As soon as curtain 1
gets across, curtain 2 has to get busy closing the gate, else each point of
the film will have gotten more than 1/250 sec. exposure.

So with this theory in mind, you see how at a speed setting of one stop
less than sync, the 2nd curtain begins its closing journey when the 1st
curtain is only halfway across the film gate aperture, thus allowing only
half the film to be exposed if a flash were used.  (A flash typically puts
out all of its light in less than 1/1000 of a second).  Depending on when
the flash goes, it may expose the first half of the frame, or the last
half, or some 50% band in the middle.

When you try to capture a single flash with two cameras, each set at the
flash synch speed, then the cameras would need to be in PERFECT synchrony
for the non-flashing camera to be able to catch the flash on the whole
frame.  This almost never happens; some small fraction of the non-flashing
camera's film is always unexposed when the cameras are set this way.

That's why Jim gets much better results when the cameras are set to less
than the sync. speed.  Even when set to only one stop more than flash sync.
speed, the cameras would need to be out of synchronization by only 1/2 of
the flash sync. speed, for some of the exposure to be lost on the
non-flashing camera.  (At that setting, the film gate is fully open for 1/2
of the exposure time.  The other 1/2 of the exposure is taken up by the
shutter curtains opening and closing).

>For the most part, though, the two cameras really do fire in precise enough
>synch so that both get flash coverage.

Jim, this last statement indicates to me that the flashing camera must be
running a little slower than the non-flashing camera - which is a desirable
condition when using flash.

To understand that, let's consider the situation when the cameras are in
what I would call "balanced synchronization."  This is the problem I have
with my new Pentax ZX-M bodies.  They are extremely well synchronized, but
not perfectly so.  To the extent they are out of sync. they appear to trade
off being first to open.  Sometimes one camera is ahead of the other, and
vice versa.

When using one of the cameras to trigger a flash, I can see (backs open,
looking into film gate) that only about half of the time the flashing
camera is fractionally ahead of the non-flashing camera.  The flash sync.
speed of the camera is 1/100.  If the non flashing camera is late by only
1/200 of a second, half of its frame is unexposed.  This is true REGARDLESS
of the shutter speed setting.  I could have them both set to 1/2 second, to
"ensure" that both get the flash.  No matter.  If the flashing camera opens
up 1/200 before the non-flashing camera, then the flash goes off as the
non-flashing camera's leading shutter curtain was only halfway open.  It
could then stay open an eternity, but still only half of that frame gets an
image from flash.

Because the cameras are "balanced" in synchrony - which is the ideal for
natural light shots - I cannot get the flashing ZX-M to be reliably late in
opening up, and therefore I am _guaranteed_ to lose 30-40% of my flash
shots.  About 50% will have a small/insignificant loss on the non-flashing
camera.  The rest of the shots are fine, because the non-flashing camera
opened first - assuming I shoot at a shutter speed lower than flash sync.;
say 1/30 sec.  (If anyone has any ideas on how to slow down one of the
cameras, delay its shutter by say 1/200 of a second, please advise us!)
That's why I cannot use my ZX-Ms in studio with flash.

On my mechanical Pentax K-1000s I solved this general problem by splicing
myself a serial flash cable, which plugs into both cameras.  This way the
electric flash circuit is closed only once both cameras have fully opened
up - triggering the flash.  It doesn't matter which camera opens first.
(Thanks are due to Dennis Hanser for informing me of this method.)  The
nice fail-safe is that if the cameras get too far out of sync., as can
happen with two mechanical cable releases, the flash stops firing.  Then at
least I know I am missing exposures, and can make an adjustment to the
release cabling.

Unfortunately I have been unable to rig either the Pentax ZX-M or the Ricoh
XR-10M this way.  The electronics do not like the "serial" twin PC cable.
Discussion on this list early last year did not reveal a solution.  I get
good flash results with my Ricohs only because one of the bodies is
"reliably" late in opening, and I can use that one body to trigger my flash
kit.  I say "reliably" because with a recent change to fresh batteries - on
both cameras - the timing apparently flipped.  Now the OTHER camera is
running late.  A little disconcerting.  I am now making certain to check
flash and camera synch with each film roll change.

Phew.  I think I've written enought to last me the week.  Hope this helps
somebody.

Boris

________________________________________________________________________
Boris Starosta, 3d artist             boris@xxxxxxxxxxxx
Dynamic Symmetry, LLC                 http://www.starosta.com
usa - 804 979 3930                    http://www.starosta.com/3dshowcase

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