84 THREE-DIMENSIONAL PHOTOGRAPHY
E is the result of perpendicular alignment but with one image
lower than the other causing a misalignment of horizontal homo-
lopes.
ALIGNMEm.--LateraZ spacing. The lateral spacing is not critical
as far as relief alone is concerned, but if the stereogram is to ex-
hibit the same amount of relief as was seen in the original scene,
it is necessary that the prints be carefully spaced. This is done by
measuring the distance between the images of the same object at
infinity, such as the tip of a tree or a distant steeple. This distance
should be equal to the distance between the centers of the lenses
of the stereoscope. However, the error resulting from a slight
change in this measurement is not obvious and in many cases does
not injure the stereogram in any way.
Often 35mm stereograms intended for projection have their
separations varied deliberately so that the separation control of
the projector need not be used during an exhibition. As will be
explained this prevents serious physical discomfort for many of
the spectators.
The separation of the unit images is then, not at all a critical
factor, its importance being. largely theoretical.
Convergence range. Lateral spacing affects the two images as
units, but in each stereogram, if there is relief, there is also a
variable convergence. If infinity and distant objects are alone in
the picture, the convergence from one to the other is slight. If all
objects in the stereogram are closely grouped, the change in con-
vergence is small, even if the group lies near the camera, but if
the stereogram includes both very near objects and distant ones,
then the convergence range or change in looking from near to
distant objects is large. This range is fixed at the time of taking
and the “remedy” must be applied before the exposure is made.
It is advisable to keep such range less than three prism diopters
when practicable.
Vertical alignment. While the eyes have a wide latitude in
fusing images of various degrees of lateral spacing, a small differ-
ence in vertical position causes an appreciable eye strain, and if
the discrepancy is more than imm, many people cannot fuse the
images at all. In good stereo this alignment should be kept within
an error of .25mm or less, and in the 35mm stereogram, the limit
is about 1/6mm (o.i75mm= %A).