P3D Re: Projection problem

[Oliver Dean <3d-image@xxxxxxxxxxxxxx>]

Hi, Tom!

Let's see if I can help straighten out the confusion:

Tom Hubin wrote:
> >snip<
> So, now I am puzzled. I would expect the stereo slides to be mounted for
> a handheld viewer. 

Correct!

> Not inverted, emulsion surface toward the viewer's
> eye, right slide directly in front of the right eye. Eyepiece lens
> allows viewer to focus on slide only a couple of inches away. No image
> inversion. Light source behind the slide.

Only one error, here; to prevent getting a mirror image, you need to
position the slide in the viewer with the emulsion side AWAY from the
viewer's eye.  As I am sure you know, if you WANT the mirror image, then
it's perfectly ok simply to turn the stereo slide around and view it
from the other side, thereby viewing what was the right eye view with
the left eye and the left eye view with the right eye, except that you
are viewing from the other side and getting the mirror view.  The
geometry of stereo lets you do this, because the mirror view also
reverses the direction of the parallax, and the stereo still looks ok
(the mirror view of the right eye image becomes equivalent to a left eye
view, etc.).

> Then the slide should be inverted for projection but not right/left
> swapped if you still want the emulsion surface on the image (screen)
> side.

This may be your area of confusion.  After the slide is mounted properly
for viewing, you invert it for projection NOT by flipping the slide from
top to bottom, which changes the side facing you, but by ROTATING the
slide 180 degrees around an axis going away from you and passing through
the center of the slide.  In other words, if you put the slide in the
viewer and it looks OK, then you put a "thumb spot" sticker on the lower
left corner of the slide surface facing you; for projection you simply
rotate the slide so that the thumb spot is at the upper right corner of
the slide surface facing you.  This surface facing you is the surface
facing the lamps in the projector.  Rotating the slide in this manner
maintains the proper non mirrored orientation, but inverts the images
AND EXCHANGES THEIR PLACES. Now, when each projector lens re-inverts the
image directly behind it, it is re-rotating that image so that it is
right side up, but is NOT exchanging its place with the other image.  So
that is how the left eye image ends up being projected by the right lens
and the right eye image ends up being projected by the left lens.
  
> So what part do I have wrong? Do you mount in a handheld viewer with
> emulsion side toward the light source? This would not produce the best
> image quality due to viewing through the slide thickness.

The film bases are so thin and so smooth in most films that the effect
of the base on the sharpness of the image is negligible unless viewed
under much higher magnification than we are using for even the best
stereo viewers and projectors.  Try reversing a good stereo slide in a
good viewer so that the emulsion is towards you, and I'll lay you odds
that you won't detect any significant difference in image quality unless
the base has been damaged in some bizarre fashion.

> Also, I would expect that if the screen were very close to the projector
> that the 62mm image seperation would require that the right image be in
> the right projector so that it is on the right side for any distance
> from 0 (screen at projector) to infinity.

Actually, for all practical purposes, not so.  Remember that, in stereo
projection, you must significantly superimpose the two images on the
screen on an axis between the two projector lenses -- the differences
that make up the stereo effect are very slight, and one rule for
projection is that for objects to appear at the distance of the screen,
their images must be superimposed by using the projector horizontal
adjustment. Accordingly, objects appearing increasingly farther in front
of the screen will have their left and right eye views increasingly
displaced from each other, right view to the left and left view to the
right.  Similarly, objects appearing increasingly farther beyond the
screen will have their views increasingly displaced right view to the
right and left view to the left (with a limitation that infinity points
should not be too far apart to fuse comfortably). These displacements
are so slight that the two adjusted images, viewed without glasses,
usually look like a single, very blurred image.  And most projectors
can't rack the lenses out far enough to focus at close screen distances
anyway, which makes consideration of very close projection distances
unnecessary.   

As a result, it doesn't matter which direction each image comes from, so
long as the system is standardized and keystoning is minimized by
projector design, distance and screen placement. Since most people want
slides mounted for viewing, the projector designers standardized their
polarization to accept inverted viewer slides when viewed with standard
glasses.

> Inversion is not a problem
> here. But you would want the emulsion side of the slides toward the
> projection lenses to avoid spherical aberration due to slide thickness.

The effect of the film base, or of the glass in glass mounted slides, is
usually negligible with respect to both spherical and chromatic
aberrations (see discussion above).

I probably made the discussion a little more detailed than necessary --
hope it wasn't too much of a chore to plow through!

Cordially
Oliver Dean