266 THREE-DIMENSIONAL PHOTOGRAPHY
stereographers who try hyperstereo use two or three times normal
base even when the object is far away. The results are of course
disappointing, Hyperstereo is without value unless used cor-
rectly.
One school of thought bases the increase upon stereo infinity.
That is, assuming stereo infinity to be 670 meters, and given a
subject whose distance is^6700 meters, a ten-times base would be
used. The fallacy is apparent. If hyperstereo is to be useful the
subject must be moved to some distance within the stereo limit.
By moving it just to this limit the whole effort is rendered useless,
for the subject continues to appear flat.
It is often said that a five times normal base is the absolute
maximum permissible. It might be cited against this that a very
successful stereogram has been made with a base of some 8000
miles, as was used in making stereograms of the moon where the
successive shots were made at a considerable time interval so that
the base was substantially the diameter of the earth.
At the same time you do not want a distant mountain to have
the appearance of being only ten feet away, for then indeed there
would be an appearance of a nearby miniature. The base must
always be selected by the consideration of the subject, and this
cannot be limited by any rule-of-thumb regulation.
The first step is to determine the apparent position in space the
subject is to occupy. But in all the computation it is necessary
that approximate values be used so that the whole thing may be
easily and quickly done mentally without recourse to involved
calculations with pencil and paper.
Our hyperstereo computations are based upon the following
assumptions which are not strictly accurate, but which have been
found from experience to serve admirably. Because of the varia-
bility of individual stereopsis, accurate computation would be a
loss of time, to say nothing of its inconvenience.
It is necessary to know the distance of the object to be photo-
graphed. It is necessary to know the distance at which this object
is to appear. That is about all there is to it. The second factor
of course may give trouble. You can assume stereo infinity to be
1000 yards, and half that the distance of best general relief. There-
fore you will wish your distant object to appear at a distance of
500 yards more or less, or 1500 feet. As all of these distances are