Ortho Stereo Reference

Creating a Standard

Everything's relative. But to what? To each other - that's what. Now we just have to agree on a few units of measure and start making declarations about standards. This applies to both capturing and viewing the image pair.

Let's measure your body parts

orthhed.gif You can put away the yardstick, we only need a ruler 2.5 inches long. Well, according to the ISO (International Standards Organization), anyway. The ISO says the standard for human interpupillary distance is 63.5mm (approx. 2.5 inches). Obviously, not everyone's eyes are exactly 63.5mm apart (including yours when you were a kid), but this is the starting reference point for establishing ortho stereo geometery. If your rebelious nature is already chaffing at such limitations, just consider that the stereo equipment and supply industry supports it. So just accept it, man.

Building the two-eyed camera

orthcamt.gif Now that we've agreed on the distance between your eyes, let's carry that standard over to our ortho stereo camera design. Here's a few rules:
* The lens base must be 63.5mm - so as to mimic human physiology.
* The lenses must be normal - normal means one that produces the same perspective as human vision (on a 35mm camera, normal is the standard 50mm lens).
* The lenses' axes must be parallel - although your eyes can converge (rotate in toward each other), doing this with your camera's lenses will introduce distortion.

Don't forget the two-eyed viewer

orthview.gif These requirements must also be carried out in the design of the stereoscope if ortho stereo is to be maintained. This means that the image must be viewed from a distance (V) that is equal to the product of magnification (M) of the object and the focal length (f) of the camera's lens: V=Mf. If you use a stereoscope (either a nice one from the '50s or the $2.95 cheapy), this math is done for you.

The Stereo Window

orthcamf.gif The distance between lenses (base) creates a location along the Z Axis (the 3rd dimension when added to X and Y) of a point which has no parallax (and thus no disparity on film). This creates a plane perpendicular to the camera lens axes, and combined with the FOV of a normal lens, a spatial pyramid can be visualized. This plane corresponds to the rectangular frame of the film, and when viewed in a stereoscope becomes the stereo window. Any objects in the scene closer than this plane will appear to float in front of the window, and any objects farther away will appear behind the window. The ortho stereo camera by default places this window approximately 2 meters (about 6 feet) in front of the camera.

Gimme space

orthwrld.gif Now that you've got a sense of the ortho camera, we need to visualize the camera within an environment. An important consideration for the stereographer when capturing an image is the distance included in the scene. Very likely, most of the photographs you take include an object located at infinity (in photographic terms this means a distance so far from a camera that rays of light reflected from an object there may be regarded as parallel). On film, the disparity of an object located at infinity must not exceed the disparity of your eyes (63.5mm). If it does, the stereo image will force your eyes to diverge (rotate away from each other). If anyone ever says, "your stereo photos make my eyes hurt" this is why (there's other reasons, but this one's the killer). Does this mean don't include infinity in your photos? Absolutely not. Just be aware that infinity plays a role in stereography. An advanced stereo trick involves "recovering" the space in images that do not include an infinity point (such as a person in a room with no windows), but more on that later.

Our theoritical world

orth3qtr.gif You've got some nice furniture, a sunny day, and a bunch of friends. The walls are transparent, but you're well dressed so it's okay. This is a three-qauarter perspective of the scene that produced the stereo image pair you'll see down the page. Take note of the location of things: the NPO (Near Point Object) is the white box; the Stereo Window is located just in front of the man; and the FPO (Far Point Object) is the man standing farthest away (off screen here, but you saw it on the previous page).

Take it from the top

topview2.gif Same scene as above but with measurement (in feet) markers. Not that you should limit your photography to sparsely decorated single room homes… this scene gives you an idea of how ortho stereo depicts familiar scale and objects. Note the camera's location relative to the room and its objects. When viewing as a stereo image, your viewpoint is be the same as the camera's.

How a Cyclops sees it

center0.gif Here's the image as produced by a mono camera lined up dead center with the scene. Stereo is not the only depth cue. You know that the men are sequentially farther away in the distance - but this is assuming the relative scale of the men has not changed (which here is not the case). Also, you know the closest man is inside the room because he is in front (and thus obscures) the wall behind him.


Insert stereo pair with measurement markers for NPO, FPO, and stereo window.


Insert depiction of horizontal ajdustment and effect of objects relative to the stereo window.

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