Three-Dimensional Photography - Principles of Stereoscopy

48 THREE-DIMENSIONAL PHOTOGRAPHY

states that true stereo is possible only when the ratios of perspec-

tive and parallax are kept constant. The principle is considerably

more complex than this simple statement indicates, but as it is

largely a matter of theory I shall not go further into it here.

It is necessary, though, to remember that perspective and paral-

lax are complementary, and in one sense they are opposed terms.

Parallax pertains to distance in depth, away from the point of

observation. Perspective pertains to dimensions in planes which

are perpendicular to the visual axis.

We ordinarily consider perspective to be essentially the appear-

ance of depth or relief, but those familiar with its principles will

recall that perspective is primarily the art of reproducing a series

Fig. 3-11. Divided word. In the stereoscope, normal vision will show

the complete word, “LONE.”

of objects at various distances, as they appear in a single plane.
The ordinary planar photograph, for example, exhibits a perfect
example of perspective, yet it is the antithesis of the stereogram
in which depth is directly and unmistakably apparent.
Parallax Controlling Factor.-The parallax, that is, the amount
of difference between the two unit images, is the sole controlling
factor in presenting the appearance of definite depth or distance.
If an object, for example a man six feet tall, is photographed at
a distance of, let us say, 20 feet, he will appear to be 20 feet dis-
tant in the resulting stereogram and will appear to be about six
feet tall. However, if between two successive parallactic exposures