CHAPTER 4CORRELATION OF CAMERA AND VIEWERRTHosTmEo.-Orthostereoscopy (true stereoscopy) is that ster-
0 eoscopy in which the objects when viewed appear in their
full life size and at their full natural distance. Direct vision is
automatically orthostereoscopic. It is also a result of a specific
method of stereogram making and viewing combined; and cannot
exist other than as a subjective phenomenon. There is no such
thing as an “orthostereogram,” simply because a stereogram which
will provide ortho results with one viewer will not necessarily do
so with another.
In speaking of “life size,” the term is used literally, but it
should be understood. Suppose you look at two men, each six feet
tall, one twice as far away as the other. You know that by the rules
of perspective, the actual retinal image of the nearer is twice as
tall as that of the farther. You can prove this by holding a ruler
at arm’s length and measuring the image height of each man
against it, but in actual experience both men appear to us to be
equally tall, to have “life size,” in other words. By experience we
see the distance, and from that unconsciously compensate for the
size discrepancy. Thus life-size means a size appearance which is
identical to that which the original object would have presented
had we been enabled to see it by direct vision.
To attain this objective it is essential that the camera and the
viewer have identical bases (lens separation); that this base equal
the interpupillary of the user; and that both camera and viewer
be equipped with lenses of identical focal lengths. In actual prac-
tice the deviation from ortho is not obvious until the viewer lenses
differ from the focal length of the camera lenses by some 30 per-
cent. The use of the standard base of 65mm does not introduce
any visible distortion even though the actual interpupillary is not
65mm. However, these are practical tolerances whose existences
have no bearing upon the validity of the theory. Similarly, in prac-
tice the camera base is often greater than that of the viewer, but
no visible distortion results.
It is obvious that to maintain orthostereo conditions renders
any attempt at conventional telestereography impossible, for the
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