HYPERSTEREO 269pillars as you look upward, It is only when you go to some ele-
vated position at a considerable distance that you can see the
characteristics of the formation.
If this scene is photographed by hyperstereo, the result will be
a stereogram including the whole of many of these pillars, and
will thus give an impression of their true nature which is superior
even to direct vision. Here we maintain the wide field of the nor-
mal lenses used at a distance, plus the exaggerated relief of the
increased stereo base.
But if there is a man standing on top of one of these pillars, and
you want to make a stereogram showing him, the original hyper-
stereo is of no value because the tiny figure is lost in the distance.
This situation calls for parastereoscopy with the combination of
telephoto lenses and telestereo base.
We cannot approach closely, because then we should have
to shoot upward and the top of the rock would hide the man.
The only point of view possible is the edge of a cliff 1000 yards
from the man. In this instance, we wish to obtain at least a rec-
ognizable image, which means that we should use, at most, an
apparent distance of 200 yards. Thus we should make use of a
lense whose focal length is five times normal, and we should use
a five-times normal base.
It will be seen that the choice of the base in hyperstereo, or the
choice of the lens-base combination in parastereoscopy, is de-
termined, not by the traditions of old time stereo, not by some
complex mathematical computation, but solely by the exigencies
of existing circumstances.
Increase the stereo base (hyperstereo) to bring the object nearer.
Increase the focal length of the lenses to make the object larger.
The two functions are not at all identical.
Let it be emphasized that there is no valid reason why the
stereographer should not vary both base and focal length of his
lenses if by so doing he is enabled to obtain a result which is
visually superior to that which would have resulted otherwise.
The freedom of choice extends beyond that limit. If the stereog-
rapher wishes to make a stereo caricature through the use of
exaggerated relief, that is perfectly good stereography. In fact, if
well done it constitutes expert stereography, because understood
theory has been used to exert a wanted control.