P3D Re: Rear screen projection, etc.

[Tom Hubin <thubin@xxxxxxxxx>]

Hello George,

> I just had the most weird phone conversation... A Mr. John Smith
> called me from Washington DC to ask about Belplascas stereo cameras.

> Anyway, he mentioned that he is has done "split frame stereo" (this
> means "beamsplitter" format) seen in rear screen projection.  He
> said that if you match the focal length of the projector lens with
> that of the taking camera lens then you eliminate any keystone
> distortion.  For a 35mm camera lens that means using a 35mm projection
> lens.  He mentioned a Bueler (sp?) 1 1/2 inch projection lens.  He is
> currently using a 4x4 rear projection screen and sits 6-8 feet away
> from it.  I asked if he uses the same device to project that he uses
> to take the pictures and he said that he has made his own device
> with adjustable mirrors to fit in front of the projector.

I visited John Smith last winter. Spent about 3 hours there. I was
interested in the technical aspects of his projection system. I got
about 1 hour of shop talk and a 2 hour show. He is very proud of his
work. His show was intersting but the subject matter is limited. That is
not a complaint. Just an observation.   

He uses a single projector, a carosel or something like that. The 2x2
images are split. He polarizes in 2 places. At the slide (I don't
remember if just before or just after the slide) and again outside the
projection lens.

This would project a stereo pair that are side by side on the screen. So
he uses 2 mirrors to project at right angles. One for the left image and
the other for the right image. The mirrors are very close to the
projection lens and act much like the mirrors on a camera splitter. He
also has polarizers on the images as they leave the projection lens. I
am not sure why this is necessary but he says it minimizes ghosting.

Anyway, the mirrors are independently adjustable. They are set so that
the left and right images overlap on the screen.

> Finally, he is aware of the Potomac Society of Stereo Photographers
> in Washington DC and he was a member some 20 years ago... but he
> calls it the Geriatric (instead of Photographic) society for obvious
> reasons (note: he sounds quite a bit old himself in the phone, I'd
> guess he is in his 80s).

Actually, I don't think he is that old. In his 60s or maybe 70s. But his
voice makes him sound much older. Very gravely. Perhaps a former smoker? 

> This gentleman sounds like a real gem.

He is definitely head strong. But his system does work and his subject
matter is very pleasing to my eyes.

I think his condenser optics and light source could be improved but it
serves his purposes. I am probably as opinionated as he is about this.
But, of course, I am right. ;)

Anyway, as hackers go, I think he has managed to put together an
interesting system.

How are beamsplit 2x2s usually projected? John Smith's is the only one
that I have seen.

Tom Hubin
thubin@xxxxxxxxx