Re: Stereo Window - Poll time!

[P3D Paul Talbot <ptww@xxxxxxxxx>]

Re: Dr. George A. Themelis requests a poll on the effect
perceived by shifting the chips.  I cast my vote for the
effect described by PT.  ;)

For me to be 100% certain, I think I would need to have two copies
of the same stereo slide, but mounted differently, in two different
viewers side by side.  I have never done this.  What I described is
the effect I have been aware of when changing how a slide is mounted.

>The brain must choose one of the two answers to explain the end result.
>I believe that in most cases it will choose my answer because Paul's
>answer requires that the size of the *known* scene is changed.

I think it requires that the size of the *reproduction* of the
known scene change.  For the size of the reproduction to change,
either the chips have to get bigger, or the magnification of the
lenses has to increase.  Neither of these occurs when you adjust
the placement of the chips.

> If the scene was really coming closer to the observer, then it should
> be getting bigger and with more depth, but this is not happening.

What George describes as necessary is true for the real world image.
Despite the incredible realism of stereo slides, they are not the real
world image.  They are a reproduction.

> On the other hand, the window is an artificial contraption...

An artificial contraption?  As your article states, it is formed
by the mask of the slide mount.  That is a very real contraption
occupying a fixed point in space when the slide is in the viewer.

> very easy to perceive it changing in size.

How can it move or change size?  The mask just sits there.

(It does change apparent size if you adjust the focus in or out,
effectively changing the FL of the viewing lenses.)

> Is the size of the objects in the scene changing too?

I have not noticed that effect.  The slide chips are the same size
they were before adjusting them, and the magnification is the same.

> How about the size of the stereo window?

No, because the slide mount is the same size it was and the
magnification is the same as it was.

> Effect of changing viewing FL:
> 
>  Lens FL    Effect Seen by both GT and PT
>  -------    -----------------------------
>  Short      O  []  Scene
>  Long       O     []      S c e n e
> 
> We agree on that.  Window moves back (it is also physically smaller,
> everything is smaller) and scene is "stretched".

I agree with your clarification of S c e n e.  I realized after I
posted that I should have written it that way.

Paul Talbot

P.S.  If you are going to participate in the poll, indicate whether
or not you wear glasses.  I do, George does not.  DrT has previously
speculated that the feeling of "being there" may be stronger for
non-eyeglass wearers.  Perhaps the difference in perception is
due to a difference in willingness to be fooled in to thinking
you are looking at a real scene rather than a photograph.


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