Re: [photo-3d] Re: Testing synch of twin cameras

[Brian Reynolds <reynolds@xxxxxxxxx>]

Alan  Lewis wrote:
> --- In photo-3d@xxxxxxxxxxx, Brian Reynolds <reynolds@xxxx> wrote:
> > > Following up on the thread about how to verify the actual synch
> > > of twin camera shutters:
> > This is a pair of photo transistors in series.  It is the
> > equivalent of a logical AND.  You will only get a signal when
> > light strikes both T1 AND T2.  This will tell you the amount of
> > overlap between the shutter firings, but not whether the shutters
> > are in sync.  Looking carefully at the diagrams above you'll
> > notice that both transistors fire for the same amount of time, but
> > offset by one time tick.  The output shows only the overlap of the
> > two signals.
> 
> Yes, that's what I was thinking.  I figured that if you knew the
> individual shutter speeds of each shutter then the best synch you
> could measure with this setup was a signal equalling the fastest
> shutter speed.  You would be synch'd at least within the difference
> between the two shutter speeds, which should be darn good.  If you
> had no signal then the shutters were completely un-synch'd.  The
> signal would gradually increase to an ideal value equalling the
> fastest shutter speed as they were adjustedfor better synch.
> 

The problem with this is that you need to know which camera shutter
opens last.  That is that camera that should have the flash on it.  As
far as I know X-sync (electronic strobe) flash triggers as soon as the
shutter is open.  If the flash triggers on the first camera to open
its shutter the flash will have shut down before the second camera
opens it's shutter.

I think that there are three things that it would be useful to know.
(1) The delay between triggering the shutter and it actually opening.
(2) The difference between the opening times of the two cameras.
(3) The delay between triggering the shutter and the flash firing.

If you are using electronic cameras with an electronic cable release
(2) should be zero.  The differences between the cameras can be lumped
into (1), variations in the delay in triggering the shutter.  If you
use all the manual settings on the camera (1) should be minimized.
Hopefully the varience in (1) would also be minimized.

> > > If the scope was able to take two signals then the transistors
> > > could be in parallel and the two shutters could be superimposed
> > > over each other.
> > 
> > This should work, but I'm not quite sure that I understand what
> > you mean by "If the scope was able to take two signals".
> 
> What I really meant is that I haven't figured out how to use my
> analog scope.  I really don't know anything about oscilloscopes but
> am trying to learn.  As is usual with stereo photography, you have
> to learn a lot of other seemingly unrelated disciplines to
> accomplish a task.  I haven't mastered the oscilloscope yet.  I've
> figured out the stereoscope, but not the oscilloscope.
> 

That's more a circuit problem than an oscilloscope problem.  You need
to design the circuit so that you can tell which photo transistor is
on without needing mutilple inputs on hte oscilloscope.  One way to do
this is by adding resistors to the circuit as a voltage divider.

Radio Shack has a series of very useful circuit booklets written by
Forest Mims III.  They're fairly small and printed on graph paper.
There is probably one on using light sensors that would have a useful
circuit.

> If this thread keeps going maybe I'd better take it to Tech 3-d.  I
> probably should have put it there anyway.
> 

Oh I don't know about that.  We haven't used any formulas yet.  I
haven't even mentioned Kirchoff's Law.  Oops.  :)

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"

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