Re: [photo-3d] Telestereoscope

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

Some of the development of the terminology stems from the development of the
devices themselves.

For example, what we refer to today as "field glasses" are simply a pair of
telescopes of usually modest power mounted side by side.  As such, these do
not alter the stereo base, and give little stereoscopic relief (impression
of depth) at the distances at which they are normally used.

What we today call "binoculars" is a short form of what were formerly called
"binocular telescopes".   Actually, the term "binocular" by itself simply
means an optical instrument that has two oculars.

The term is also correctly used to identify the oculars (lenses that we look
through) in a stereo microscope, a binocular microscope (which has two
oculars but only one field lens), and even our own eyes.  When I refer to a
field lens singularly, I am referring to a single optical lens system.  Such
a "field lens" may be composed of more than one lens element, but if so they
are all coaxial.

The word "ocular" originally was a medical term applied only to biological
eyes, but later was extended to lenses that we look through to differentiate
them from lenses or lens elements elsewhere in an optical system.

Stereographers have extended the application even further to refer to any
stereo base, as in "interocular", although to be absolutely correct this
should be called "interaxial" (distance between the optical lens axes), and
interocular reserved for the distance apart of lenses we look through, as in
stereo viewers.

Since the term "interocular" has come into such common usage for general
stereo base applications, and few people seem to understand "interaxial", I
often use "interocular" when talking to other stereographers, rather than
the more correct term.

The binocular telescope is an instrument quite different from a pair of
field glasses, in that it also includes sets of prisms which have two
functions.  They shorten the physical length of the instrument through
folding of the optical path (for this purpose they are sometimes used in
monoscopes as well).

Perhaps of more importance to the stereographer, they also extend the stereo
base, allowing for improved depth perception at their normal working
distances.

The telestereoscope was simply an earlier term for what later became known
as the binocular telescope, and in my mind is a more descriptive term.

Even though we have shortened "binocular telescope" to simply "binoculars",
we still refer to them by a still longer term "a pair of binoculars" as in
"a pair of scissors".

A pair of binoculars is, however rather redundant, as the prefix "bi" means
two, or a pair.  If we still said "telestereoscope" it would be reasonably
short, and certainly descriptive of the instrument.  Turning the subterms
around, it might be even easier to understand if we called them
"stereotelescopes".

This last word has seen limited use in recent popular literature in
reference to some of the very large, very wide base astronomical telescopes.

However, here, to imply that they are stereoscopic due to the wide stereo
base is really stretching the meaning.

Astronomical distances are so great that absolutely no stereopsis can be
detected by the human eyes, as a result of the very wide stereo base alone
(except, perhaps, within our own solar system).

Rather these instruments are dual primarily for greater light gathering
power and resolution (one instrument cancels some of the atmospheric
distortions of the other).

Stereopsis at astronomical distances is attained through either or both of
two methods.

Time parallax, in which two images are photographed through a telescope at
two different times, allowing the earth to move within the solar system, and
the solar system to move within the galaxy.

The other is computer evaluation, such as depth maps created from other
depth information such as red shift (Doppler effect), etc.  This has
resulted in some very spectacular stereoscopic astronomical imagery.

JR

----- Original Message -----
From: "Dr. George A. Themelis" <drt-3d@xxxxxxx>
To: <photo-3d@xxxxxxxxxxx>
Sent: Saturday, October 21, 2000 11:00 PM
Subject: [photo-3d] Telestereoscope


> >call it a telestereoscope.
> >
> >Checking my dictionary, I couldn't find this term, but on the
> >net I found numerous references to it
>
> Gabriel, forget the net and open F. Drouin's "The
> Stereoscope and Stereoscopic Photography" written
> in 1894 :)  (Available from Reel 3-D, everyone
> should have this book and read it!!!)
>
> Page 25 shows a picture of a Telestereoscope. A child
> is using in a scenic view, as we use mounted binoculars
> today.
>
> Drouin writes: "The Telestereoscope, invented by
> Helmholtz, 1857, is an apparatus the function of
> which is to increase the relief of distant objects
> by enlarging the optical angle..... " etc.
>
> George
>
>
>
>