Ever since Charles Wheatstone discovered the principles of stereoscopic vision in 1838 and photography was invented a year later, people have been combining the two. 3D or stereoscopic imagery uses two images of the same scene taken from slightly different viewpoints. When viewed side-by-side, the images are combined in your mind, producing the illusion of depth. To make this easier, viewers were developed, the first by David Brewster, and one of the best by Boston's Oliver Wendall Holmes. Views of foreign places, and other scenes of all kinds, became the rage in Victorian homes and millions of stereo cards were sold. You can easily find some of these views in almost any antique shop although some are very expensive and are fast disappearing as collectors gather them up. These mass-produced stereo cards have two prints mounted side-by-side on a piece of cardboard. To see the 3D effect you insert the card into a holder on the viewer and then view it through a pair of eyepieces.

Capturing Stereo Images

Almost all-stereo images start with a pair of photographs taken from vantage points a few inches apart, duplicating the spacing of our eyes that gives us stereo vision. Although there are stereo cameras with two lenses and rigs that let you use two cameras, most digital photographers capture a pair of stereo images with a single camera and lens.

• Tripod mounted slider bars keep the camera aligned as you capture a pair of images. You take the first image, slide the camera to a new position and take the second. Because of the time between shots you can only capture static subjects. It's important that between pictures the lens focal length not be changed, that the lens be kept exactly parallel, and the camera be moved just the right amount— called the stereo base distance—normally about 2.5 inches (6.35 cm) or so.
• Beam splitters are attachments that mount on the front of a lens and use mirrors to capture two images on the same frame during the same exposure. Because both are captured at the same time, you can use one of these splitters to capture moving subjects. Beam splitters work only on normal focal length lenses (50mm or so) and only on lenses where the front filter ring into which it screws doesn't rotate as you focus.

  When taking stereo images, here are some things to keep in mind:

• Keep track of left and right images, perhaps by always shooting left images first.
• Use a small aperture for maximum depth of field so everything in the image is sharp. If you can't get enough depth of field, be sure the background is so out of focus that it isn't distracting. For small objects, put a sheet of cloth or paper behind the subject to eliminate an out of focus background.
• When taking close-ups, use longer focal length lenses.
• Photograph scenes that have depth. In landscapes, there should be something of interest in the foreground.

Displaying Stereo Images

Once you have captured a pair of images, there are several ways to display them in print or on the screen.

• Stereo pairs are mounted side-by side and viewed with a stereoscope or other viewer. With practice, you may even be able to view them unaided by such devices. All you have to do it look at them in the right way. When you finally see the depth information you have "fused" the images. When practicing these unaided techniques, take a break if you experience eye fatigue or discomfort. Keep in mind that not everyone can do these techniques.

A pair of images designed for cross-eye viewing is arranged side-by-side with the image for the left eye on the right, and the one for the right eye on the left. To view the images above in 3D, hold your head level and stare at a point between the two images and slowly cross your eyes. The images should merge so a third image appears in the middle in 3D. The original images will still be seen on either side, so ignore them and concentrate on the middle one. You should see the darker (blue) circle floating above the lighter (red) circle.

If you can't get the effect, hold your index finger half way between your eyes and the image so both halves are still visible. While staring at your fingertip, slowly bring it closer and farther from the screen. At some point, the images should merge into a single 3D image.

A pair of images designed for parallel viewing (also called relaxed viewing or free viewing) is also arranged side-by-side. However, the image for the left eye is on the left, and the one for the right eye is on the right. To view the images, place your face close to the images and then slowly back away while staring "through" the images to an imaginary point behind them. When there appear to be three images, concentrate on the middle one until it becomes 3D. If you first see four images, concentrate harder on the imaginary point behind the images until they become three images. It may help if you rotate your head slightly to the right or left while keeping it level.

• Anaglyph images. Du Hauron, a French scientist patented the anaglyph method of stereoscopic photography in 1891. Anaglyphs use a pair of colored images, usually red and blue, superimposed over each other with one image offset slightly from the other. When viewed through a pair of glasses with different colored lenses, usually red and blue, the image appears in 3D. These are sometimes called "pure anaglyphs". Some full color images can be used for anaglyphs but most won't work very well. There are many software programs available that create anaglyphs and the glasses are available on line.
• Shutter glasses have high-speed electronic shutters that open and close in sync with the images on the monitor. When the left image is on the screen the left shutter is open and the right shutter is closed which allows the image to be viewed by your left eye only. When the right image is on the screen the right shutter is open and the left shutter is closed. Since this process happens fast enough, your brain thinks it is seeing a true stereoscopic image. If this shuttering speed is not fast enough, you can still see a stereoscopic image, but you may also see some flickering.
• Animated GIFs. You can give an illusion of stereo on the Web by putting the left and right stereo images in an animated GIF. The effect is almost the same as you get with shutter glasses, but since both of your eyes are open, the animation seems to wiggle. You can also use the Flash format for the same effect and it does give you better control of the frame rate and compression.
• Autostereo Images. The problem with most 3D viewing systems is that they require some form of eyewear. However, there are systems that dispense with these by using various techniques to guide the right and left images into the correct eye. These autostereoscopic displays are expensive and display the images in a format that is squashed side-by-side. One big problem with these systems is that you can only view the images from a specific angle. The 3D image isn't seen if you are not positioned correctly. Also, since the two images are interlaced on the screen at the same time, the resolution is only half of what it is with other methods.
• Pulfrich Images. Strange as it may sound, if video is shot with the camera moving to the left or right, or if an object is spinning, it can be viewed in 3D. To do so, you cover one eye with a dark filter and leave the other eye uncovered. Known as the Pulfrich effect, this technique was used for an episode of the TV show "3rd Rock from the Sun" in May of 1997. With enough research you may be able to locate a copy of this episode (it's the last show in the second season). You view the Pulfrich sections with one lens of a pair of dark sunglasses held over your right eye.
• Polarized Images. A final way to view stereo pairs is to display them on the screen one after the other with different polarizations. The first image is displayed with vertical polarization and the second with horizontal. When wearing a pair of glasses with matching polarizations, each eye only sees the image that matches its polarization.