P3D Projection and the "window"

[Project3D@xxxxxxx]

In a message dated 26/06/98 21:47:14 GMT, you wrote:

<< Date: Fri, 26 Jun 1998 17:29:35 -0400 (EDT)
 From: boris@xxxxxxxxxxxx (Boris Starosta)
 To: photo-3d@xxxxxxxxxxxxxxxxx
 Subject: P3D toe-in two 2x2 projectors?
 Message-ID: <v01530501b1b98ba5a83f@[207.69.169.8]>
 
 In digest 2805, I wrote:
 
 >Many apparently good people have contributed to the thread(s) concerning
 >projection standards, whether or not to adjust during the show, etc. etc.
 ...
 [I plan to set a floating stereo window close to observer...]
 >As I will be showing 2x2 slides, this setting of the stereo window will be
 >achieved by simply toeing in the two projectors a bit.
 ...
 >Why do I want to do this?  As many of you know, I shoot almost exclusively
 >close-ups, and even with controlled depth, the disparity in my slides - if
 >they where "mounted to the screen" - would likely put many points beyond
 >infinity!  Plus, I would like my presentation to be more close to
 >orthoscopic, meaning the images should be placed much closer to the
 >audience than the screen itself.  (In essence, I want to do what they do at
 
 I've been behind on my digests, and rushing my reading.  I didn't notice
 any comments on my plans.  Is that because I've planned to do something
 conventional!?  How horrible!>>

Hmmm. I must have missed this first time around. There are two very
significant issues in projecting stereo slides.

Firstly, NOTHING the projectionist does will affect the placing of the window
RELATIVE to the SCENE.

Adjusting the horizontal separation on the screen - either by sliding the
lenses horizontally in a conventional stereo projector or by toeing in
standard 50 x 50 projectors will move EVERYTHING forwards and backwards -
relative to the screen.

SO, the window is ALWAYS set at the mounting stage. After all, the film chips
are (or should be) firmly affixed to the mounts. The projectionist cannot
change their relative positions. So your stated aim of  

<<setting of the stereo window will be achieved by simply toeing in the two
projectors a bit>>

is clearly a non-starter, and arises due to a misconception in the basic
nature of the window. You MUST set it at the mounting stage. Projection simply
arranges for your complete composition to be presented simultaneously to an
audience greater that the single user of a stereoscope / viewer.
 
 <<If this sort of projection is common, by what standard are projected slides
 adjusted?  I thought adjustments were always made to set the stereo window
 at the screen.  Does the projectionist instead try to judge infinite
 points, and set them to be 2.5 inches apart on the screen?  What if there
 are no points at infinity?>>

Well, you thought wrong! Most projectionists do actually cheat a little and
tend to superimpose the frames on the screen, thus setting the window on the
screen, however, they should be separating the infinity points at a 65mm
separation. This will have the effect of placing the window on the screen for
a projected size of 54 inches wide for a Realist format slide. Most of us use
larger screens than this, and the effect is for the window to float in front
of the screen. "what if there are no points at infinity?" - well, as you have
set up the projector to place the infinity points at virtual infinity, if
there are no infinity points in the scene, then everyhting will be closer than
infinity! This is why it is BAD PRACTICE to adjust the projector for each
slide. It is only done because many slides presented for projection are not
mounted correctly, and the adjustment is made to made the slide "viewable".
But it does nothing to put the window right, or fix the vertical errors
(relative to the mount). You still get all the horrible floating edges (and
the author carries on in blithe ignorance!)
 
 <<I do need to figure all this out before August!
 
 Boris >>

Well, I hope some of the above has been useful. I think you said that some of
your subjects are macro / close - ups. In this case, nothing will be at
infinity (by definition!) BUT you still have to work within the limitations of
the system. The "window at 7 feet" rule comes from the geometry of the Realist
camera, and give a difference in separation between infinity and near points
ON THE MOUNTED SLIDE of 1.2 mm. So, if your infinity points are separated by
63mm, your close points should be at least  61.8mm apart. With close - ups, if
your furthest detali is separated by 63mm, then the closest should be more
than 61.8mm unless you want them to extend through the window. In this case,
they should not pass over the edge of the frame or they'll look unnatural
(because it can't be done with a real window - you know, one of those things
with glass in them!).

There is absolutely nothing to stop you from floating the window in front of
the screen, but make sure your room is REALLY dark (or the actual screen will
be seen and slightly disturb the illusion) but you need to do it with
moderation. 

Hope this helps!

Bob Aldridge
Stereoscopic Society Projectionist


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