[photo-3d] Re: screen shapes

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

Since I have no experience with projection, I usually sit out these
discussions - but I believe I have an argument to make from the standpoint
of theory.  I'm curious to see if it holds up under practical weight.

Regardless of width, the various (usual) 35mm stereo formats (4P, 5P, 7P,
8P) have one thing in common: the height of the image.

In general, the larger the projected image, the better (up to a point). 
When a square screen is used, the 4P and 5P images would, in the best
case, be made to occupy the entire vertical area.  Changing to 7P or 8P
would require a smaller image height - lengthening the projector focal
length or somehow moving it nearer the screen) to allow the full width of
the image to fit.  That means less magnification of those images. 
Assuming that audience and the screen must stay in fixed positions, this
creates more "stretch" in viewing the wider formats than in the narrower
ones.  I am guessing that it also imposes a different infinity spacing.

With a wide-format screen approximating the 8P aspect ratio, all images
may be projected the same height - ie: with the same magnification -
minimizing projection-imposed variations from orthostereo when changing
between formats.  Of course the other elements of shooting focal lengths
and the room setup will come into play in how ortho things are, but
between formats the situation remains equitable.

When I worked as a movie projectionist as a young man, we did have films
with different aspect ratios to project on the same screen, and that is
how the problem was addressed.  When a narrower format was employed,
light-absorbing black panels were moved in at either side of the screen to
provide a nice uniform black border around the image.  A similar masking
method could be simply employed for the vertical formats in stereo.  (I'm
sure this has been done.)

What say, gents?

Bruce

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