HYPERSTEREO 265normal vision at a distance not exceeding 250 to 300 yards, and
the parallax for an object at 100 yards is preferable.
Parallax is angular movement, and a given amount of par-
allax is that corresponding to a certain angle. It can easily be
understood that for some definite angle, let us say one minute of
arc, the base at any given altitude (distance) must have a definite
value. If we double the value of the base, we either double the
value of the angle, or we double the value of the altitude.
This means that if we double the normal stereo base, the dis-
tance and relief of all objects in the field of view are given double
value, objects appear to be only half as far away, and appear to
have double their usual depth. It requires but a moment of con-
sideration to see that when the apparent distance is halved and
the apparent relief doubled, the specific relief of any object is
exactly the normal degree of relief for the new apparent distance.
That fact, so often overlooked, is the reason for the great success
of hyperstereograms. There is no distortion of specific spatial
values within the stereogram itself.
The practice of hyperstereoscopy is simple. Most stereo cameras
are provided with a single lens cap for this purpose. The left lens
is covered and an exposure is made. The right lens is then cov-
ered and the camera is moved to the left through the required
base distance minus one base length, and the second exposure is
made. Inasmuch as the motion is often a matter of some feet, the
exposures are made freehand, aligning the camera as accurately
as possible for the two shots. When the negative is developed, it
should be cut apart and the bases aligned carefully. This is the
one really difficult factor in hyperstereo technique. Naturally, as
the two exposures are successive, the subject should be motion-
less, as it almost always is in hyper shots. If a single-lens camera
is used, it is moved sidewise through the base length desired.
When using 35mm cameras and color film, the camera must be
carefully leveled and some key object placed in exactly the same
position in the finder for each exposure. Cameras with positive
interlocks are used by making the usual dual exposure, moving
the camera and making the second. Later mount the two lefts to-
gether, then the two rights. This gives two complete hyperstereos.
This brings us to the critical point of the discussion, that of the
determination of the degree of base increase to be used. Most