STEREO PROJECTION 185tion between focus and convergence, there is little wonder that
most people refer to it as “exaggerated relief” simply because they
only associate it with nearby objects whose relief is such that it
cannot be encompassed all at one time by their eyes.
The rebuttal is that when there are no closeup objects, this
trouble does not exist. Therefore it is suggested that for projection
purposes, nothing nearer than ten feet be photographed.
Yes, the argument is true. It is also true that if you make a ten-
nis court ten feet square with a net six inches high anyone could
play good tennis-but both beg the question by reducing the stand-
ard of existing conditions to meet the limited physical ability of
undeveloped and untrained muscles and coordination1 The anal-
ogy is a good one, Bonded focus-convergence is a condition of
physical unfitness.
Here is the way it works. The screen will normally be viewed
from a distance of about ten feet, and if the images of objects at
ten feet are registered then both focus and convergence are normal
for the nearby object. When the object is at 50 feet or just about
15 meters, the convergence is not quite a half diopter while at ten
feet it is barely over one diopter. In short to see objects within the
10-15 feet range, the amount of convergence change is only about
i:200, and the slight separation of focus and convergence can be
achieved unconsciously.
But when the object lies at only 30 inches or about 750mm, the
convergence required is about 1:zo or 5:ioo which means that
looking from an object in the distance to an object 30 inches away
requires ten times the convergence change required to look from
50 to 10 feet1 And that ten times increase in separation of conver-
gence and focus is a lot. You cannot do it without feeling the ef-
fect. And that feeling is just what the spectator is afraid of because
he thinks he is injuring his vision. In fact it is the same pain asso-
ciated with the use of any unused set of muscles or unusual mus-
cular action.
Those who have been in stereo for some time usually acquire
this separation spontaneously. They are also familiar with many
of the peculiarities of normal vision unnoticed by others. As a
result they are not affected by the so-called “exaggerated relief.”
- As a test remember that if the exaggeration is visible ONLY in