Re: Stereogram on the "Stereo Window"

[P3D Paul Talbot <ptww@xxxxxxxxx>]

Author/editor/professor George A. Themelis, in explaining the
stereo window, writes:

> The stereo window is controlled by changing the borders of the images.<snip>
> In slides where a fixed mask is used (as in Realist slides) bringing
> the chips closer together pulls the window towards the observer.                                             ^^^^^^^^^^^^^^^^^^^^
> Pulling the chips apart pushes the window further back.

I find this part of the (otherwise very useful) discussion very
confusing, due to the part I marked.

Adjusting the spacing of the chips alters the appearance of objects
relative to the stereo window, as any experienced slide mounter
knows.  But which is moving, the objects in the scene, or the
window itself?  Some might say it doesn't matter, it's all
relative.  It is true that the movement is relative between the
objects and the window, but the article states that the window
is moved relative to *the observer.*  I have never been able to
interpret the visual effect in this manner.  The *apparent distance
of the window* from my eyes does *not* appear to me to change when
I adjust the chip spacing.  However, if I look at a slide in a RB
and then look at the same slide in a viewer with longer FL lenses,
such as the $3 viewer, with the longer FL lenses it *does* appear
that the window moves relative to my eyes.  In fact it does move,
because the slide must be moved farther away from my eyes to use
the longer FL lenses.

I'll try to diagram the effects, using O to indicate the position
of the observer, [] to indicate the window, and "Scene" to represent
the objects in the scene.

   Chip
 placement  Effect                
 ---------  ------               
  close    O    []Scene
   far     O    []   Scene

Adjusting the chip placement moves the apparent position of the
scene relative to the fixed window.

 Lens FL    Effect
 -------    ------
 Short    O  []  Scene
 Long     O      []      Scene

The longer FL causes both the distance from the observer to
the window and the distance from the window to the objects in
the scene to be stretched.

Paul Talbot



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