A Two-mirror Stereographic Beamsplitter
This 2-mirror "beam splitter" creates single-frame stereographs by directing the image from
one mirror into the left half of the frame and the image from a second mirror into the right half.
The two images create a stereographic pair because the mirrors are viewing the same scene from different angles.
The two mirrors need to be placed at a slight angle to each other. (If the mirrors are aligned parallel to each other,
the view through the camera looks like a single flat mirror with a line down the middle,
regardless of the separation between them.) Turning the left mirror slightly toward the right one allows it to
reflect the same scene into the camera, but from a position that's a few inches to the right, relative to the scene.
The left mirror can be rotated to align the images: this is not a fixed angle because the
necessary angle between the mirrors depends on the amount of zoom that is used. (The camera here is an Olympus C-730
with 10x optical zoom, and this arrangement allows me to take photos over the full range.)
The pivoting bolt is directly below the right edge of the mirror, so that edge stays in the center of the frame when
the mirror is rotated. The smaller mirror can also be tilted so that the two images can be aligned vertically,
but once set, this does not need adjusting for different shots.
Perfect alignment is not necessary horizontally or vertically since the images can be digitally adjusted
and cropped, but best results are obtained when the alignment is very close.
The two scenes in the resultant camera image are mirror images, of course, but also the "right eye" view is on the
left, so the entire frame needs to be flipped right-to-left after uploading. Also, because of the
mirrors, the images can use a little sharpening and contrast enhancements. These can be done either before or
after alignment.
This 2-mirror method does cause perspective differences in the two images, which if not corrected makes them hard to view even if the horizontal and vertical alignments are correct. The perspective difference is similar to what you would get if you took a picture, then turned the camera to the left such that the scene that was in the left half of the frame is now in the right half. (More precisely, it's as if you also moved the camera a few inches to the right before turning it.) This difference in the angle of the lens relative to the scene causes "keystone distortion," a difference in perspective that can cause eye strain when viewing the stereographs.
But fortunately, this perspective difference (and the alignment) can be corrected automatically by StereoPhoto Maker, the great freeware program by Masuji Suto. The image can be opened in SPM as a "stereo view" (which assumes a pair and splits the image into halves), and one click of the Auto alignment function (in the Adjust menu) does everything necessary to make a perspective-corrected, nicely aligned stereo pair.
I used first-surface (front-surface) mirrors because a little experimenting showed that ordinary back-surface mirrors introduce too much distortion. I got these FS mirrors, 2x3" and 5x7", from HighReflectiveMirrors.com for $15.50 plus $10.00 shipping. Smaller mirrors could be used -- and the device would be more compact -- by placing the mirrors closer the lens (and by using a camera that doesn't stick the lens as far out in front of the body as this C-730). I used a 1/4"x20x2-1/2" bolt through the balsa wood block to mount the camera, and I glued a block of basswood to the bottom, sized and shaped like the tripod's camera mounting plate so it fits securely on the tripod head.
... and the results.
All of these shots were taken on my first outting with the camera, and one thing I've learned is that it's important to get the camera precisely aligned with the mirror dividing line, and also to get very close to having the same scene in both halves. Otherwise, much of the width of the images has to be cropped off.
> Larger pairs Requires parallel viewer
Click to join photo-3d