T3D Message to the Public

[Peter Abrahams <telscope@xxxxxxxxxx>]

Here is another essay I wrote for CSC's big show.  This one discusses the
hazards & pitfalls of viewing stereo photography.

When viewing a presentation of stereo photography, there are a few notes
that should be kept in mind.  A few percent of the population are stereo
blind, and an additional percentage find stereo images to cause eye strain.
 If this applies to you, then there is no need to subject yourself to
discomfort, simply close your eyes or find the exit.  For the vast majority
of people, the beauty and information carrying capacity of stereo imagery
is a very rewarding experience.  However, the audience must keep some
guidelines in mind.  If an image seems uncomfortable, it might have been
knocked out of alignment, and if you try to force your eyes to focus on it,
you can easily give yourself a headache.  Just close your eyes until the
next slide.  If you feel any discomfort or strain while viewing, don’t
force the issue, again close the eyes for a bit.
There are a number of reasons why stereo images can cause eyestrain.  If
you look at a (single) picture of a mountain on a wall that is 15 feet in
front of you, your eyes will focus to a distance of 15 feet, and will
converge slightly so that both are looking directly at a point on the wall.
 If stereo photos include a very distant object (a mountain), the two slide
projectors will place the left & right images of the mountain 65mm apart on
the screen, so that your eyes will be looking straight ahead, like they
would when actually viewing a distant object (65mm is the approximate
average distance between adult eyes).  This means that one part of your
visual system is seeing an object at 15 feet, and another part is seeing it
at a great distance.  Few people are bothered by this, but in the
inexperienced, it can cause confusion in the visual system, which can lead
to strain.
Another cause for eyestrain is the sequence of stereo photos, which ask
your eyes to focus at a distance for one slide, then perhaps much closer
for the next.  A long sequence of photographs can cause fatigue.
Finally, the slides used for projection must be mounted with great
precision, and misalignments to a fraction of a millimeter are perceptible
on the screen.  An occasional tilted image is inevitable, whether from a
slide that has been dropped or one that is difficult to mount.
It is uncharacteristic for an activity to be introduced by noting all the
problems that can arise in participation.  However, something like one in
ten people can not or do not enjoy stereo imaging, and they must not feel
left out (for they can still use one image), and also need not broadcast
dire warnings about headaches and discomfort.  Those of you who are about
to experience stereo imaging for the first time should be aware that it
might not be suitable for you.  The great beauty (and usefulness) of stereo
images are of such that we hope you will give 3d a try.

_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx
the history of the telescope, the microscope,
    and the prism binocular


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