Re: [photo-3d] Tri-Delta Beamsplitter

[Olivier Cahen <o_cahen@xxxxxxxxxxxxxxxx>]

From: Olivier Cahen <o_cahen@xxxxxxxxxxxxxxxx>

	Several postings were discussing about a special beamsplitter this
week.
Sergio Baldissara noticed it last Monday and said it was too expensive.
Mark Shields thought that it was equivalent to an ordinary beamsplitter,
tilted by ninety degrees.
Peter Davis replied that such a tilted beamsplitter would never give a
stereo image (it is obvious).
Lawrence Kaufman remembered that such a device is not new.
Sheldon Aronowitz identified the beamsplitter and said that it is much
better than the other ones, but without explaining why.
So did George Themelis who added more information.

	I will try to explain why this one is better.
	With a simple beamsplitter such as Pentax, you get a huge keystoning
that can never be compensated for projection, except with two projectors
very far away from each other AND with slide mounts cut in shape of
keystones. In other words, it is completely impossible to project the
images without heavy trouble.
	In this beamsplitter, each image is rotated by ninety degrees, so that
each other image is upside down. Instead of shifting the left and right
homologues
in opposite directions by the keystone aberration (due to perspective
viewing from the center of the lens to each side of the film), both
deviations are in the same direction, so that there is no more vertical
displacement between the homologues. 
The images taken with such a device are therefore suitable for
projection, unlike the ordinary beamsplitter.
Olivier Cahen, Editor of the French Srereo-Club Bulletin.

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