244 THREE-DIMENSIONAL PHOTOGRAPHYhyperstereo, and in micro work where a fraction of a base is used.
For anything between 62 and 70, the practical result is satisfactory.
Rotary paraZZax.-The usual stereo technique involves the use
of parallel lens axes. However this rule is violated often enough
to make the exceptions interesting. When parallel axes are not
used, convergent axis are substituted so that the camera positions
resemble those of the converging eyes. The explanation as to why
this convergence is inferior to parallel axes is given elsewhere in
this book.
We have seen that physically the convergent camera and the
rotated subject are essentially the same, so that the convergent
camera becomes essentially a phase of rotary parallax.
In speaking of the convergent camera, it has been assumed that
the camera is used for successive exposures and bodily rotated be-
tween the exposures to converge upon the subject. However, with
the rigid stereo camera and simultaneous exposure the technique
would seem to be impossible because the stereo camera will not
bend. (It may be of interest to know that a convergent camera
was made commercially which was adapted to the binocular micro-
scope objectives made for the Greenough type of stereoscopic
microscope.) It is possible however to adapt rotary parallax or con-
vergent axes to the conventional stereo camera. This is done by
bending the light beams rather than the camera body.
In the paragraph above supplementary prisms are indicated.
In practice the magnifying lens commonly used for close-up work
is combined with a prism in a single piece of glass. Thus when
such a supplementary is applied to the stereo camera lens, there is
a simultaneous change in focal length and a deviation of the axial
ray. The prism power is so adjusted to the magnifying power that
when the object is focused it will also be centered in the two films.
Such supplementaries are made in three close-up powers and a
fourth weak prism is supplied for those who wish correction at
distances less than ten feet but within the focal capacity of the
unassisted camera lens.
As is true of all “rotary” parallax, objects at infinity will be
abnormally displaced, due to the crossing of the axes at the object,
but this does not interfere if the background is kept reasonably
near.
However, there is one factor which must be considered, one