APPLIED STEREOSCOPY 207process was introduced, the final steps in the optical development
almost coinciding with the initial efforts to produce the grid
stereogram.
The parallax found in all images formed by lenses is often
overlooked, but it is one of the fundamental factors in all photog-
raphy. Only the pinhole has practical freedom from parallax, all
other photographic images having a certain degree of inherent
stereo parallax.
As first pointed out by Brewster, if a lens whose diameter is
more than 65mm be provided with two apertures situated at the
ends of a horizontal diameter of the lens, and two images are
made by alternately covering the apertures, those two images form
a normal stereo pair.
It follows that if this is true of two portions of the lens separated
by the normal stereo base, then any two points in a lens will form
images which will exhibit parallax corresponding to the loca-
tions of the two apertures. It is known that when a stereo pair is
made with a separation as small as a half inch, the pair will
exhibit a visible degree of relief, so it follows that any lens in
ordinary use must form an image in which there are definite
stereoscopic pairs of images.
If three test objects are set up in a line receding from the cam-
era, and if the lens is focused upon the central object, the images
of the other two objects will be blurred, This is normal experi-
ence. However, if the lens be provided with two relatively small
apertures and the same picture made, the central image is still
sharp, but the other two now show two overlapping, relatively
sharp images instead of the original blur. Thus the photographic
image really consists of an infinte number of infinitely sharp
images, which appear collectively to be sharp or diffused depend-
ing upon the degree to which they are superimposed in exact
register.
It can be seen then, that among the other images, a pair ex-
hibiting stereo parallax is included in every photographic image.
It is also apparent that even where the image is “in focus”-that
is, exactly superimposed-the superimposed images have parallac-
tic differences, hence cannot be superlatively sharp. The larger
the diameter of the lens, the greater these differences and the