CORRELATION OF CAMERA AND VIEWER 73grams with a greater than normal base. It is a practice which
has been almost universally condemned, simply because it is a vio-
lation of the principles of orthostereoscopy. However, as the image
is not proportionately distorted, the subject has a natural appear-
ance, thus making the technique one of the most potent available
to the stereographer.
Because of the arguments involved and the importance of the
subject, it will be fully discussed in a later chapter, instead of
trying to deal with the subject in the limited space available in
the present chapter.
Convergent Axes.-The argument is often offered that the best
stereogram results when two cameras are used, converging in the
manner of the human eyes. This argument, although quite valid
in special fields such as photomicrography of objects in a single
plane, is a result of faulty reasoning. The stereoscopic camera is
not a substitute for our eyes. It is a tool, an instrument which pro-
duces an imitation of the original scene toward which our eyes
function just as they would with the real scene. Our eyes still con-
verge normally when the stereogram is made in a parallel axes
camera.
The objection to the convergent axes system is that a very
definite spatial distortion is produced, and the greater the degree
of convergence the greater the distortion. When the camera axes
are parallel, the homologous distances of all images are directly
related to their real distances, but when convergent axes are used
this essential relationship is lost, and apparent distances vary
widely from the original ones. Any degree of convergence intro-
duces distortions of distance, and hence is not satisfactory unless
the subject matter is confined substantially to one plane.
The convergent stereogram (and the divergent as well) can be
made and can be viewed up to a limit, but they are never truly
realistic.
Figure 4-16 shows two cameras converged upon object A. When
viewed, as shown in Fig. 4-17, we find A is satisfactory, but B and
C are apparently farther away than their true positions, b and c.
It should be noted that the farther the object from the cen-
ter of convergence, the greater the distortion. Thus convergent
axes result in a variable degree of spatial distortion,
Only parallel axes will produce uniformly satisfactory stereo-