One interesting aspect of 3D photography is the wide range of different techniques that are used to obtain the pairs of images that make depth-rendering possible.  For many normal real-world subjects the desired lens separation mimics that of human vision, where the sensors, i.e. the eyes, are about 2.5 inches apart.  However, variations from this are often useful or even required.  When recording distant objects an increased separation helps recover depth perception that is lost with normal vision.  When photographing tiny subjects a very small lens separation is necessary.  For example, in low-power macro-photography the basic problem is that the desired lens separation is typically around 1/30'th of  the distance to the object (using a "normal" focal length lens).  If a flower interior is 90mm (3.5 inches) away, the two stereo images should be made with lenses roughly 3 or 4 mm apart.   Stereo photo-microscopy (or high-magnification macro-photography) has additional depth-of-field and depth-perception issues.  Digital image processing can be used to overcome many of the problems that arise in this latter area.

Some techniques that have been used successfully in various areas of 3D photography include:

1.  Horizontal displacements of a single camera on a slide bar for normal and macro work.

2.  Special 2-axis lenses for close-up photography.

3.  Mirror-based splitters and Mirror-Lens Combos for macro 3D.

4.  Digital processing of image stacks for photo-microscopy.

5.  For general stereo photography, several web-guides are available:   George Themelis (off-site), and Mike Kersenbrock (off-site), among others, have posted helpful info on 3D imaging.