A B O U T  B I N O C U L A R  V I S I O N
widened version of the article
published from the Italian astronomical magazine IL CIELO, October 1997


The idea of this article has been born from the ascertainment, made nearly by chance, that in the examination of two-dimensional images (photographs, designs, stamps, etc) is possible to catch from them a very greater information content (shadings, details, plasticity) if the shrewdness is had to observe them in a particular way.

It is useful to remark at once the importance that such fact covers for the amateur astronomer, since it allows he to take advantage not only of his own sight, but also of the power of his instrument.

This fact gives me the occasion in order to make a general speech on the human vision, and to bring to the acquaintance of the readers aspects not unknown, but surely little known.

The HUMAN VISION is by nature binocular. The origin of such characteristic could perhaps be attributed to the general tendency of the universe to produce symmetrical shapes. But one can not exclude a certain "intelligence" of the nature, that would have so found, in a three-dimensional world, the way to render perceptible the third dimension.

It would be wrong, however, to think that the combined vision of the two eyes is only necessary when it is about to perceive the third dimension.

This is, in fact, not always possible, you want for the excessive distance of the objects (but we will see ahead as it can be obviated, within certain limits, to such disadvantage), you want because a third dimension simply does not exist (two-dimensional representations of the reality).

That said, we try now to analyze in detail the several ways to observe the reality, that are fundamentally three:

  • monocular vision;

  • binocular stereoscopic vision;

  • binocular non-stereoscopic vision. 


This is the less suitable and less convenient way to observe. It is generally adopted out of need. Only in rare special cases can replace with advantage the binocular vision (ex.: view-finder of the gun).

A technological problem, initially not resolved (ex.: Galileo’s telescope), and other economic and logistic problems, are to the origin of the monocular vision, that is totally unnatural and susceptible to cause problems to the observer, conditioning in fact his visual capability. The perception of the details and the shadings turns out reduced and the image does not have plasticity.

In the case of the amateur astronomers, such reduced capability limits of fact the exploitation of the full power of the instruments, with the consequent, not always justified run to always greater openings, that often doesn’t resolve the problem and creates new ones.

The amateur suffers such limitation, beyond that for economic and logistic reasons, also for one kind of laziness, and accepts it in fact as something "natural" !

It would be favorable to fight such tendency, sensitizing interested peoples on the enormous advantages of the binocular vision.


The binocular vision, broadly speaking (that is not always and not necessarily stereoscopic) is the sole that allows to perceive in integral manner the content of information of an image. The stereo effect is always useful, and sometimes indispensable; to such point, that even certain animals realize it, and, in case of necessity, they catch up or increase it moving laterally their head. 


In its natural shape it happens with the two eyes. The vision, regarding that one with a single eye, turns out remarkably improved, and not only for the stereo-effect, as we will see more ahead.

Such vision can yet furtherly be improved with the aid of special tricks. The more common example is that one of the classic prismatic binoculars, that, having their lenses at a distance greater then the interpupillar distance, increases the effect. In other derived instruments, the base can be varied within certain limits.

In photography the purpose is caught up:

In astronomical field, interesting results can be obtained:


taking advantage of its libration (shots in the same phase but in different lunations);


taking advantage of its rotation. Due to its angular velocity, an interval of only 15 minutes between two shots is sufficient to supply stereoscopic images, able to show even differences of level in its atmosphere. This is possible because a rotation of Jupiter of a due angle is equivalent to a translation of the Earth in its orbit of the same angle, occasion that allows to have a basis as great as one wants.

Analogous speech can be done for Mars.


taking advantage of the phase of its rings. In this case, however, in order to perceive the stereo effect, it is necessary to observe the pair of images of the planet with its pole axis in horizontal position.


Taking advantage of their movement. This will not evidence any relief, but allows to perceive the comet in a first plane and the star much far away.

Analogous speech for asteroids.


Different is the situation in the two-dimensions world. We refer mainly to the photographic reproductions, not necessarily stereoscopic.

Despite the photographs are, normally, monocular representations of a two-dimensional or three-dimensional reality, it is not sufficient to observe them with a single eye. To watch them with both eyes is the natural way; it gives remarkably better results, compared to what can be obtained with a single eye. But it continues to supply, as strange it can seem, a still limited vision!

EXISTIS YET a special way to observe the images, stereoscopic or not, that allows an enormous and unsuspected gain, able to supply a really integral vision, not only of the details (some cent of mm), but also of the most tenuous shadings, that pass usually unnoticed.

It is not easy to understand and to explain as and because it happens; one is induced to think that every eye sees to its way, in a partial and incomplete shape, and that only the combined vision of both eyes, through the integration produced from the brain, is able to supply a full and complete image, where nothing can escape. The photographs acquire an extraordinary plasticity, and even eventual minims differences between the two images (nova, variable, asteroid etc) jumps immediately to the eye, replacing the classic "blink-comparator"! If you have nothing else within reach, you can try with the well known illustrations of the puzzles magazines ("find the differences").

The main techniques in order to observe these image-pairs, are they stereoscopic or not, are three:


The viewer is a simple device, equipped with two lenses. In the flea-market it can happens to find some very beautiful, of the beginning of the past century. There are apparatus for slides and for paper-copies. Who didn’t find it, can easy construct one, but one must have a certain manual ability and have two identical lenses, of appropriated focal-length (approximately 25 cm.). Realized this, remains always the need to respect, and eventually to can vary,  the interpupillar distance, what places limits to the horizontal dimensions of the photos (the distance of their centers can not exceed the one of our eyes).


When it is about images published in books or magazines, one resorts often to their reproduction in two complementary colors, and to their observation with special glasses, what involves generally the loss of the color.
Nevertheless, with the use of a special technique, it is possible to avoid, at least partially, the loss of color, using three colors instead of two.


The method of the intercrossed vision is to prefer to the viewer that, among other things, is cumbersome and not always within reach. Given that the crossing of the optical axis of our eyes is a natural what, and not only natural, but also closely tied to the mechanism of curving of the crystalline, one looks so the photos:

We give obviously for discounted the need of a good lighting and the absence of troublesome glares; in case these are present, is generally possible to eliminate them. And we give also for discounted the possession of a normal sight; otherwise it will be necessary to use appropriated glasses, above all for the correction of an eventual astigmatism.

The overlapping of the two images (if necessary with little of exercises) happens in automatic way; and in equally automatic way it stops on the demanded position, as soon this is caught up.

The distance of the two photographs (from the observer) can be varied within wide limits, like also their dimensions (contrarily to what happens with the viewer). Besides, this method offers the ulterior great advantage that is not necessary to respect the interpupillar distance.

In (at) the moment in which the two images are overlapped, our eyes see three:
two lateral ones, that are not taken in consideration, and a central one, formed from the overlapping of both images, that is the sole that is examined. If the photographs are made from two different points of view, it will be possible to perceive also the third dimension.


When it is about slides, the third dimension can be perceived without the loss of the color resorting to special projectors equipped with polarizing filters, and to special glasses, also polarized.

When it is about images in motion, in black and white or in colors, the third dimension can be perceived taking advantage of the movement and the persistence of the images in the retina, also in this case with the aid of special glasses.

In the TV, for example, whichever transmission can be seen in relief; it is not necessary that it is a special stereoscopic transmission. There are yet strong limitations, since the perception of the relief depends on the sense of the movement. If the special glasses are made in order to see in relief subjects that move from left to right, they will not work in the event of subjects that move from right to left !

That said, it does not remain to me that to say you "have e good time"! (enjoy yourself!)

Before concluding, I would want to draw the attention of the readers on the usefulness of these techniques not only to astronomical scope but also on the convenience to use them for other purposes, like the observations of stamps and coins, (where it becomes easy to evidence eventual differences), and, above all, of reproductions of art-works. Try with some illustrated postcards, or with the copies of your astronomical photographs.

But if you have nothing else within reach, two cigarette packages can do the job, giving you mono-images (the illustrations) and stereo-images (the boxes).

db - Rome, July 1996



If two objects L and R are placed to different distances and aligned with the observer, farther than a reference point C, the right eye sees the far away on the right of C and the left eye sees the far away on its left.

If of such objects two photos are obtained, made from the point of view of the single eyes, the stereo-effect can be reconstructed.

Such photos can be obtained with various techniques:        

For the examination of such photos, there are special viewers "ad hoc", that allow to observe the
photographs from near, maintaining parallels the axis of the eyes.

In lack of a viewer, one can resort to the "intercrossed vision", putting the right photograph
on the left, and vice versa. The intercrossed vision has advantages regarding the viewer, since it allows to observe also very bigger photographs, arranging them at a suitable distance.

If the photographs have been obtained with a stereo-camera that reproduces them both on a same plate, they could be observed with the viewer only after separating them, to the aim to straighten them separately (the lenses, in fact, forms turned-over images). If one does not want to separate them, and they straighten turning over the whole plate, the right will be found on the left, and vice versa, and their observation will be possible only with the intercrossed vision.


1) in the passport-photos made with instantaneous Polaroid-type apparatuses, equipped with 4 lenses (that turn over every single image, and not the whole plate of 4 ) straightening the whole plate without to separate the single photos, those obtained with both right lenses will appear on the left, and vice versa, and will be possible to have correct stereo-images only resorting to the intercrossed vision. For their observation with the viewer, it will be necessary to separate them.

2) observing the photos obtained with Polaroid-apparatuses, it could be waited to see mirror-images, what does not happen. How never?
The explanation is in the fact that, in reality, the Polaroid-prints are observed not from the side in which cast (form) the images produced from the lenses, but from the opposite side, as it would happen in a ground glass. In the development process, in fact, the part of the emulsion where the image casts becomes opaque, while that posterior one, where the reversal happens, becomes transparent.

3) What happens, instead, in the special stereo-cameras, like Exacta, that have a stereo finder that serves also as viewer? (attention to not confuse the two functions! ).

Given that in phase of observation with such stereo-viewer (that does not turn over the images), both 18x24 mm photos must remain close in the 36x24 mm slide, each one in its correct position (not turned over, not mirrored, and with their reciprocal positions not inverted), the necessary right/left reversal of their positions is obtained directly with the lens, that is in fact a set of two lenses, of which the right forms a turned-over image on the left part of the frame, and the left a turned-over image on the right part. Straightening the frame, each image will turn out to its correct place.

4) in the observation of the so obtained photos, the reality comes faithfully reconstructed. In the view-finder of the camera, instead, the photos appear mirrored, because of the reflection in the camera-mirror.


Suppose to observe a glass-slide, containing two images of 60x60 mm each. Printing said images on paper - that can be made for contact – the photos will be able to be separated and to be arranged correctly for their observation with a viewer. Its original, instead, could be observed only with the intercrossed vision.

But what can we wait turning-over the plate, or watching it from the side of the emulsion? There are obviously four ways to observe it:

In all the cases the stereo-effect will remain in the correct way, since the mutual position of the image-points does not change. In other words:

- case a in equivalent to observe the scene normally;
- case b is equivalent to see the scene reversed;
- case c to observe it mirrored;
- case d to observe it mirrored and reversed.
The stereo-effect, clearly, cannot go lost


It must be avoided to observe the images in incorrect position.
If the photo that must be to the right is on the left, and vice versa, a certain stereo-effect will be visible, but with paradoxical results:
if it is, for example, about two equal spheres on two different planes, the farthest will appear nearer, but smaller, while the nearest will appear farther, but larger, and the observer would see a scene apparently "realistic", but it will interpret it in mistaken way (as if the farther sphere were several times larger).

In the event of a wild beast that approaches threatening the observer, instead, a paradoxical situation will be had, in which the tail of the animal will appear nearer of its teeth!


When stereo-photo are published in a magazine, especially if they are in colors, it would be opportune that they are printed in such an way that one can observe them with the intercrossed vision, since this is a method within reach of all and does not need any instrument.

One can not start from the principle that the reader is incapable to do it !!!

In fact, not all people has a viewer, and the use of glasses with the complementary colors involves generally the loss of the color. 
Beyond to that, as already pointed out, a common viewer places limits to the horizontal dimensions of the images (distance of the centers of the photos not greater then the interpupillar distance); the sight, in fact, can be parallel or convergent, but not divergent. 

Exists naturally the way to obviate also to this disadvantage, by means of the construction of a special stereoscope for the examination of photos that exceed the mentioned limits, like this one here reproduced, constructed from Luis Retegui, professional photographer in La Plata, Argentina, equipped of own illumination system.

But it also has its limits, as far as the dimensions of the photos, and is in any case something very special, which very few can have, unless they decide to construct of one…