The World in 3D

Stereo Vision

Of course the world stretches out in three dimensions – so obviously that most of us take our everyday 3D experience for granted. But depth perception is not at all simple. Going back at least to the ancient Greeks, people have wondered about the relation between human vision and solid objects (our word “stereo” comes from the Greek for “solid”). It wasn’t until 1838 that the English scientist Charles Wheatstone demonstrated that our perception of depth is formed in the brain, not the eyes.

Offset a few inches apart in space, our two retinas receive flat and slightly different images from photons bouncing off the same object. Information passes via the optic nerves to the visual cortex where the brain examines the two data streams for minute differences. This mental analysis forms the basis of a three-dimensional model which the brain projects into space such that the model locates in the identical position occupied by the original solid object. When everything works right this model exactly maps to reality; the real-time 3D projection emanating from our head allows us to move about the world without having to think about where to place our foot next (and without stubbing our toes too often).

Stereo vision is a learned behaviour. It appears in infants around twelve weeks after birth, when they begin clapping their hands and reaching for objects – actions that establish a connection between sight and touch. 3D vision continues to develop along with motor coordination well into elementary school years.

3D Imagery: The Optical Illusion

Normally when we look at a flat picture or screen our eyes see exactly the same image and so our brain perceives no depth – the third dimension is absent. But a stereogram can create the illusion of 3D by presenting each eye with a slightly different image. 3D glasses or other special viewing devices allow each eye to see only its appropriate left or right image. The brain receives the visual information and kicks in with its usual depth-determining calculations. We experience the sensation of great space even though the flat object we’re actually looking at (the stereogram) is only inches in front of our nose.

How to View a Stereogram

Place the image pair flat on a table or before you on the screen. Look through your 3D viewer and relax your eyes as if you were staring off to outer space. Move closer to or farther from the images until they come into focus and merge into a three-dimensional scene. An inexpensive foldable paper 3D viewer is available from Loreo -- we've gotten good results from this one.

Another method that may work if you don't have a viewing device: Orient your head so that your nose points at the dividing line between the left and right images. Stare at them until your eyes "glaze over" -- totally relaxed, looking beyond the stereogram out to infinity. Eventually the two images will merge and the third dimension will pop through the flatness. Mastering this technique takes practice. Enjoy the illusion!

How to Make a Stereogram

It takes two pictures of the same scene, with some side-to-side separation between the two (the exact amount depends on how close your subject is and what effect you’re trying to get). The simplest way to do this if you have only one camera is to use the “cha-cha” method: Take the first photo while leaning slightly to your right side, then shift to your left (doing your best not to rotate your camera or change its vertical position) and take the second. Note well: this method only works for stationary subjects! There are some nice programs available for processing digital images for 3D – we use StereoPhoto Maker to help us align and crop stereo pairs, then print our stereo cards from Photoshop.

If you want to make 3D view of objects in motion, you’re going to need a special lens attachment, a purpose-designed 3D camera, or two cameras attached to a single shutter-release trigger. These days we mostly use a portable rig we put together with two mirrorless Olympus PEN E-PL8 compact cameras. A nice feature of this model is that the LCD monitor flips both up and down, which is handy for aiming the cameras while holding the rig high above your head or at waist height in the “rest position”. These cameras have zoom lenses, and we’ve learned the hard way to always check that both units are set to the same focal length. Another tip is to set the display of one of the cameras to always show the horizontal level indicator. This lets you know if you’re holding the two cameras askew – if they’re too far out of alignment in the horizontal plane, their images won’t line up well when you go to make the stereogram.

"Binocular vision is the Trinity of transcendent physical perception:

the Father, the right eye; the Son, the left eye; and the Holy Ghost, the brain."

-- Salvador Dali