P3D Bruce's Challenge - III

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

We have defined our three recording variables (F, B, I) and the way
they affect the metric (measurable with a ruler) characteristics of 
the recorded image.  We have also defined our two viewing variables 
(Fv, Bv).  Now we will explain how the viewing variables affect the 
perceived stereo image:

- Increasing the ratio B/Bv (mainly by increasing B since we cannot
  change Bv so easily!) makes the image look closer to the observer 
  and smaller in size.

- Increasing the ratio F/Fv (either by increasing the FL of the
  recording lens or by reducing the focal length of the viewing lens)
  leads to "squash" or compression of depth (distances are shortened
  in the z-direction)

These effects are VERY DIFFICULT to explain with words and are not
quantified well with geometrical models because our BRAINS use
learned experience when interpreted the observed image.  Different
people see these effects in different degrees.

But, I will attempt an intuitive explanation of why things seem to
look that way:

By increasing the stereo base you introduce more depth.  More depth
usually means closer.  So things look closer.  They look closer but
their on-film size has not changed.  So they must be smaller.
Another way to see that:  If the stereo pair is recorded with a stereo
base wider than the eyes, upon viewing the observer is transformed 
into a giant whose eyes are spaced wider apart (to match the camera
lens positions).  But it is usually easier to think of the scene
shrinking into a small model than thinking ourselves grown into
giants.

By increasing the focal length of the recording lens you make it
appear as if the subject is closer.  However, both the perspective
and the parallax (deviations) have not increased because in reality
we are still far away from the subject.  The result of this conflict
is perceived depth compression.  This effect is also seen in 2d but
only the perspective is working for us.  For example, most of us are
familiar with races where when the runners are filmed face-on, they
appear to be lined up (depth compression) and we are surprised to see 
how far away they really are when the camera changes angle of view.  
The reason we see the runners lined up is that they all have the same 
size (no perspective) which is caused by filming the scene from far 
away (with long lenses but it is not the lenses but the distance that 
has eliminated the perceptive).  This factor works for 3d too but the
stereoscopic deviations (which are minimized due again to the long
distance) are a stronger factor in 3d observation.

To be continued...

George Themelis