LET US BUILD A
Giorgio Carboni, April 1996
English version revised by Ed Vogel
The stereoscope is a simple optical instrument which allows us to see in relief
drawings and photographs. For us, to see in three dimensions a solid body, is a fact so
natural that we do not even take it into account. Also it is normal to see
"flat", that is to say lacking a third dimension; drawings or pictures. Did you
ever ask yourself why this occurs? The phenomena is so natural and obvious that many
people do never pay attention to it. For trying to understand how this happens, make this
experiment: while observing a solid object, as a bottle or a chair, cover an eye with a
hand. Even in this case, the image will lose third dimension. It is true that some clues
remain, as shapes and the variation of size with distance, which help you to have an idea
of volume of things and their position in the space. Nevertheless, using an eye only, you
lack depth perception. If you want to touch an object, it can occurs you are wrong.
Removing the hand that covers the eye, you will see return the third dimension of the
space and of the objects which are on it.
The ability of viewing bodies in relief is defined stereoscopic sight. It is due to the
interaction of the two eyes. Each eye watches an object from a different direction by
rapport the other, and get from it a slightly different image. The geometrical differences
between the two images are used by the brain for drawing the perception of deepness. This
skill is been developed particularly by the predatory animals which must evaluate
precisely the distance of the prey. Primates, from which man descends, developed
stereoscopic vision for the necessity of evaluating exactly the position of the branch to
which grasp during moving on the forest trees.
Now it should be evident why drawings and pictures are lacking of depth: this occurs
because both the eyes receive the same image. Since there are not geometrical differences
between what each eye perceives, the brain cannot extract the information of the third
Is it possible to give the volume to drawings and photographs? Is it possible to view
in three dimensions imaginary objects as drawings? Is it possible to obtain the volume
starting from two pictures both of them flat? Of course, and you can get very impressive
stereoviews. In this section, we will see how it is possible to take 3D pictures and,
above all, we will see how build some apparatus for their observation.
HOW TO TAKE 3D PICTURES
For making photographs in three dimensions, you need two images of the same object, but
they must be taken from a different direction the one from the another. You can keep the
corner which separate this two views similar to the one formed by the eyes, or even
greater (hyperstereoscopy) for objects poor in plasticity. For obtaining 3D pictures,
professional photographers use special cameras with two objectives. If you have not a
camera of this type, especially designed for stereophotography, you can use also a normal
camera. As showed in figure 1, you need take two subsequent pictures by displacing a
little the camera between one shot and the other.
||While you made so, move the camera parallely, keeping it
pointed towards the main subject. Take care avoiding vertical differences in the framing.
Obviously, this system is right for steady objects only. For taking 3D photograph of
moving things, you need to use two cameras, to shoot together. You can also to use an
accessory to fit on the objective of a camera in order to transform it in a stereoscopic
camera. In this manner each 24x36 mm image on the film, will be divided in a couple of
24x18 mm stereograms.
You can see in 3D also drawings. For obtaining that,
you need made two perspective or axonometric views from two different directions. When you
are drawing with the computer (CAD), you can make this operation easily, by obtaining the
two stereograms simply rotating the drawn object.
METHOD AND INSTRUMENTS FOR STEREOSCOPIC
The last figures of this article have been obtained with methods as those we described.
Now, we have the two images (or stereograms) for the stereoscopic view, how can we observe
them in a suitable way? You must see at the same time both stereograms, one by the right
eye, the other by the left one. It is not easy to see a thing with an eye and another
thing with the other eye without to be inured to it (a chameleon would look at us with
commiseration). It is at this point that the stereoscope enters in the field and shows its
importance. The stereoscope is a simple optical apparatus which make easier the
reassembling the two stereograms in an unique tridimensional view.
Thereafter we will deal with four techniques for observing stereograms:
1 - with naked eye (images up to 6x9 cm);
2 - with a simple stereoscope (images up to 6x9 cm);
3 - with stereoscope for slides;
4 - with a mirror stereoscope (images of great size).
BY NAKED EYE
Keep side by side the two stereograms which should not exceed 6x9 cm. Put the couple
close to the eyes, in contact with your nose. At this moment each eye sees the stereogram
that is in front. As a consequence, you see an unique image, extremely blurred. Do not try
to focus it. Move the sheet with the stereograms 10 cm away from your eyes and stop. Now
you should see three images, very out of focus. Every eye sees two of them, but the
central one is due to the fusion of the two "internal" ones. If the central
image should dissociate a little, wait relaxing the eyes until the two parts overlap
exactly. Now, slowly move away again the sheet until the central image will be distinct.
If the central image tend to dissociate, make a pause and wait that it overlaps. When you
will see the central image clear and well fused, it will appear to you in three
dimensions, as hanging in the space. It will seem to you that the support of paper on
which the stereograms are lying is vanished. The effect is astonishing. The first time, to
succeed in the task of viewing a stereoscopic image without using instruments is
difficult, but the subsequent ones will be much easier. If this is the first time you try
it, make a copy in a reduced scale of drawing, in order to facilitate the goal.
Remember that scientific magazines as Science and Nature often report stereoscopic
drawings of molecules. This magazines do not give instruction on the method for their
observation, as great is the familiarity of the readers towards the stereoscopic
representations. In fact, most of those people are able to observe stereoviews even by
naked eye. The use of reading spectacles can facilitate this operation.
A SIMPLE STEREOSCOPE
There are people who do not succeed in the task we described and, after a lot of
useless attempts, they abandon. Especially for these person a stereoscope (fig. 2) is
useful. It consists in a very simple instrument, in which the central wall and the lenses
facilitate the superimposition of two parts in an unique 3D image. The stereoscope we have
shown features a removable bottom. This allows you to put the instrument on a magazine
carrying stereograms, without have to cut it. The stereoscope is also disassemblable for
facilitating its transport and conservation.
You can mount on the instrument two magnifying lenses with a focal length of 30 cm, for
people of normal sight. Myops prefer weaker lenses or nothing. On the contrary,
hypermetrops prefer stronger lenses. A stereoscope with lenses is easier to use, but they
are not indispensable. In fact the muscles which drive the convergence of the eyes and
those which regulate focusing, even if they are distinct, are accustomed working together.
While observing stereograms, we ask to the eyes to keep parallel, as they were watching
something distant, whilst they must focus at 30 cm of distance. The magnifying lenses make
easier this operation because they allow to keep eyes adjusted to the infinity while they
are observing near objects. Lacking these lenses, you can use reading spectacles as well.
In any case, many people are able to use successfully the stereoscope even if it lacks of
lenses and without using spectacles.
A STEREOSCOPE FOR SLIDES
||How can you view stereoscopic slides you obtain by the
technique showed in figure 1? You need built a stereoscope for slides. It is an instrument
very simple to carry out: it is a question of putting two single slide-viewers side by
side (fig. 3).
As you can see, the two viewers are mounted on an opened
stirrup in order to allow modifying their distance and fitting it to the interpupillary
distance of the observer. Notice the plastic stripes glued under the right viewer for
keeping it aligned. A screw placed under each slide has the function of regulating the
framing in the vertical direction. This method is the simplest to use among those
described. With it, even people which have difficulties with the first two method
described, are able to observe stereoscopic images.
A MIRROR STEREOSCOPE
|Using the stereoscope of figure 2, the images cannot be
larger of interpupillary distance (about 6 cm), otherwise the eyes would be forced to
diverge. Our eyes easily can converge (observation of near objects), or at the most to
stay parallel (observation of distant objects), whereas they have great difficulties in
diverging. Hence, for observing stereograms of large size, you need of a different
instrument, as the one showed into figure 4.
This stereoscope is made up by
four mirrors. It could work also with the two central mirrors only, provided you agree
that the stereoview appears to you specular by rapport to the stereograms. If you want
avoid this disadvantage, you must prepare the stereograms in their turn specular by
rapport the original. But this is an useless complication which you can avoid introducing
in the instrument two other mirrors: the lateral ones, which erect the image.
The mechanism of sight in relief is innate and works even in people who use it rarely
or never such as squinters. Conversely there exist also normal people, not squinting, who
do not possess the tridimensional sight of reality. Simply they have not this ability that
for other people is so normal as to be easily forgotten.
Here some stereoscopic images. Instead of seeing them directly on the monitor, print
them and reduce their size so that between the homologous points of two stereograms you
have not more than 6 cm: the normal interpupillary distance. It is better you make the
first attempts with a copy in a reduced scale. For the stereoscopic viewing of this
images, follow the suggestion of the paragraph 1.
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