268 THREE-DIMENSIONAL PHOTOGRAPHYdefeat the purpose, as the large object would have its image spread
beyond the limits of the negatives.
It must be remembered that the conventional telescope does
not, as so commonly stated, bring objects nearer. It simply in-
creases the perspective values, the angular value of the field of
vision filled by the object. Therefore we shall make use of long-
focus lenses only when the object is too small to be satisfactorily
recorded from the nearest accessible camera position.
If we maintain a proportional increase between base and magni-
fication, the type of reproduction closely approximates that of
orthostereo. We reproduce the objects at their full natural tele-
scopic size, and with that distance and degree of relief which is
fully natural for that size. Thus if the lenses are doubled in focal
length and the base is also doubled, objects are shown in the size
and relief normal to a distance which is just half that of their true
distance.
If the object is small and far away, the proportionate increase
just described, that is, the use of parastereoscopic technique, will
have much the same effect as that of using a telephoto lens upon a
planar camera-except that we have stereo relief. That is to say,
we are shown the object as it would appear from a point of view
midway between the real camera position and the object. The
same distortions appear as in the planar telescopic record, but like
them, these distortions are extremely difficult to perceive even
when they are pointed out in detail. Any distortion as subtle as
this may be completely disregarded without detriment to the qual-
ity of the stereogram.
Therefore, in making stereograms of objects at a great distance,
we first determine the base necessary to introduce the desired de-
gree of relief. The next step is to select the lens which will cause
the image to occupy the desired space upon the film, and if it is
desirable that strictly normal proportions be preserved, the first
two steps are compromised so that the two increases are of the
same degree.
For example, suppose the scene you wish to photograph is a
group of chimney rocks, tall pillars of stone as found in some of
our great National parks. These are huge things, and when you
are among them, the general effect is lost because your effective
vision is held by the immense bases or the height of one or two