T3D Re: Advice sought on home-made beam splitter

[Brian Reynolds <reynolds@xxxxxxxxx>]

Dwib wrote:
> I would like to make my own beam splitter and I wonder if any of you
> out there have any experience with them. I may be mistaken, but my
> notion of a beam splitter is a device that allows you to make stereo
> exposures with a single-shutter camera by dividing and expanding the
> perspective of a given lens. The cheapo Argus stereo camera seems to
> employ this principle. I've seen them also in kits for Polaroid Land
> cameras. They seem to be little more than a simple housing with four
> mirrors, each arranged to gather light from two separate
> perspectives and then reflect it until it can enter the lens as a
> united stereo pair on a single frame of film. My thought was to
> build one for use on a 4x5 camera. If I were to cut a darkslide in
> half length-wise, each pair would then be comprised of two 2x2.5
> images. Of course, the business of making a medium format viewer
> would have to be undertaken, too, but I'm thinking that such things
> have been made successfully by more than a few participants of this
> list. If there are web sites which explain the design and
> fabrication of the sort of beam splitter I'd like to make, please
> steer me their way. If there are kits available, I'd like to hear
> about them.
> 

If you are gong to build a beam splitter with mirrors (as opposed to
prisms) make sure you use first surface mirrors.  These mirrors have
the reflective coating (usually aluminum) on the front of the glass.
An everyday mirror has the reflective coating on the back of the
glass.  Light striking the glass before the reflective coating will
cause additional reflections.

If you have a laser pointer and an additional mirror you can set
things up to use the mirror and laser to align the mirrors for the
beam splitter.  In the diagram below the slashes ("/") are the beam
splitter mirrors, the underbar ("_") is the test mirror, and the other
lines represent the light path of the laser (which starts and ends at
the "*").  The laser beam leaves the pointer, hits both beam splitter
mirrors, hits the test mirror and then goes back through the beam
splitter and hits the pointer.  Adjust the beam splitter mirrors (one
at a time) until the reflected beam re-enters the orifice on the
pointer.  Be very careful using a laser with mirrors.  Stray
reflections are a serious problem.  The laser must be pointed exactly
90 degrees to the test mirror.  The easiest way I can think of to
ensure that is to use a large test mirror.  Align the pointer with the
test mirror without the beam splitter in the light path.  The reflected
beam should go right back into the pointer's orifice.  Now put the
beam splitter in the light path and adjust it.

                    *
                    |
                    |
                 /--/
                  |
                  |
                 ____


You might have better luck building a beam splitter with prisms.  At
least then you only have to adjust the alignment of one half with the
other, and not the internal alignment of each half of the beam
splitter.  You'll need pretty big prisms for this to work.  Check out
American Science & Surplus <URL:http://www.sciplus.com/>, or other
surplus dealers, for old tank prisms.

By the way, I think beam splitter is probably the wrong name for this
device.  When I think of a beam splitter (especially in relation to
lasers) I think of a half silvered mirror or a prismatic cube.  Does
anyone know if there is another name for the device stereo enthusiasts
call a beam splitter?

I don't know that you actually want to bother with a beam splitter.
If you cut the 4x5 darkslide lengthwise you can get two 2x5 exposures
on a sheet of film.  (Actually they'll be about 1-7/8 by 4-3/4.)  If
you put the camera on a slide bar (the Bogen sliding quick release
plate works great) you should be able to make some nice stereo pairs
using two separate exposures.  Slide the camera to the left and raise
the back so that the lower half of the film holder (aligned
horizontally) is centered behind the lens.  Put the cut dark slide in
the camera and take an exposure.  Slide the camera to the right and
lower the back so that the top half of the film holder is centered
behind the lens.  Reverse the orientation of the cut darkslide.  Take
the second exposure.  Now print the negative so that the objects in
the top and bottom half of the print are 4 inches apart.  You should
be able to view this with a View Magic over and under viewer
(available from Rocky Mountain Memories
<URL:http://www.frii.com/~rkymtmem/>).  Note that your camera will
have to have back rise/fall.  Many flatbed cameras do not have (or do
not have enough) rise/fall.  A monorail camera should be fine.  You do
not want to substitute lens rise/fall for back rise/fall.  Doing so
with change your viewing point.  If you have enough horizontal shift
you could do this without using a slide bar.

I haven't tried this myself mostly because I don't use cut sheet film
holders.  I don't have a darkroom to load holders so I use Readyload
and Polaroid.  I am thinking about building an 8x10 camera along these
lines (I'll wind up getting a film changing tent) so that I can
directly contact print the negative.  I have also thought of mounting
a View Magic viewer behind the lens board to allow simultaneous
exposures.  I've built a camera out of foamcore to use as a testbed
with (non-stereo) pinholes.  When I get a chance I'll build another
for stereo.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"


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