To understand the art of three-dimensional viewing - or seeing things in depth (not to be confused with the totally misused term of '3-D graphics' so beloved of computer buffs where they merely mean 'perspective drawings' ) - you need a basic understanding of human depth perception.
This doesn't require a degree since, in its simplest form, it's just the two different views that our eyes see every day of the week. Because of the placement of our eyes...usually on either side of our nose...each eye sees a slightly different perspective. This difference is minimal, but it is enough to allow the brain to add and subtract the difference and present it to us as a view with depth from front to back, and side to side. Move your head, and the perceived images change. DO IT YOURSELF
Try this test. Hold up one finger vertically in front of your nose about a 30 cm (or 1 ft) away from your face. Now close one eye at a time, and you will be immediately aware that each eye is seeing a different view. And that's all there is to it! That's the cause of your stereoscopic vision.
To translate this into a reproducible photographic image, it should be immediately obvious that you must aim to take two pictures that try and mimic the different in distance between your eyes. It is for this reason that early experimenters in photography - right back into the early 1800s - often started out with two lenses on their primitive cameras. There are many excellent examples of 3-D photography dating from the late Victorian era and early days of the 20th century. Anyone who has seen the IMAX production of 'Across the Sea of Time' will have seen some truly remarkable enlargements of these stereo photos which are outstanding even though they are all in black & white.
Given its long history, it is not surprising that 3-D has thrown up many different systems for achieving the effect of a solid three-dimensional image from flat images. Some of these ideas have been fine and have stood the test of time, many have fallen by the wayside. Even within my short span of 3-D knowledge, I have been presented with a variety of ideas where the 'inventor' thinks that he perceives a stereo picture - but nobody else can see it! Some of the prototypes I have been asked to develop have ranged from technically excellent ideas (but where the cost of development could never be recouped...a sort of 3-D Millennium Dome) to the totally impractical that have obviously provided hours of fun for the originators but could never hope to go anywhere. AND THE BETTER IDEAS?
However, not all 3-D ideas are wild and woolly, and some of them work extremely well. Most have even stood the test of time, while more recent developments have benefited from those earlier efforts. Here's a few current in production examples that spring to mind. The Loreo Stereo camera
Using ordinary print film it allows anyone who has ever pressed the shutter button on a camera to produce quite outstanding (in the quality sense as well as the dimension) images without requiring an ounce (gram) of technical 'know-how' . Shoot a roll of film, have the prints processed, pop them into the viewer supplied and, 'Viola', 3-D! The ViewMagic viewer
Simpler and cheaper, even, than the Loreo camera, it lets you use any (yes ANY) 35mm or APS camera to get those in-depth pictures once you understand the modus operandi i.e. how to do it. The viewer is sold complete with two excellent booklets on the subject and - provided there is no movement in the subject you are photographing - will produce the clearest 3-D you have ever seen.
And even cheaper still, and utilising a similar method of shooting as the ViewMagic, is the Pinsharp Viewer. Unlike the aforementioned systems, this is intended for the production of stereoscopic slides (one for each eye, of course). Again, this has the advantage of using any 35mm or APS camera. TWO PHOTOS ARE A 'MUST'
However, the one thing that all these systems have in common is that you must take two photo's, each from a slightly different angle...the secret of good stereo. Anyone who comes to you suggesting that they can produce 3-D images from one photo should be shown the door at a fast rate! It takes two eyes to see 3-D and needs, at the very least, two pictures to reproduce those results in a viewable in-depth form. There's one notable exception and that is the hologram, but holography is far more complex, and not within the remit of this series of articles.
Of course, in a piece as short as this I've only just touched on what is a truly vast subject. Over the years many books have been written about 3-D and how to obtain the best results, most of them are now out of print, mainly because - in my opinion - they were so complex that they turned people off stereoscopic photography rather than making it readable and enjoyable. One book I saw had pages and pages of equations and calculations relating to optical separations, infinities, and a host of other considerations that you have no need to worry about. No way am I prepared to assault you with these ideas. I will try and keep it simple.