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P. A. Stonemann, CSS Dixieland |
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CSS Dixieland
Probing the depths of knowledge
These essays by P. A. Stonemann, CSS Dixieland, cover a wide range of
historical, philosophical, scientifical and technical subjects. Each page
deals with a particular topic, divided into sections and explained by itself.
Every page shows at its top hyper links to every other page. The Start page
also has short descriptions of the other pages. CSS Dixieland expresses
gratitude to the readers that make this work meaningful.
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Stereoscopy page
How to make three dimensional images
Sections in this page
How to make Stereoscopy
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How to make Stereoscopy
"La vue stereoscopique est la chose elle-meme, elle rend tangible la Nature,
avec la profondeur de ses different plans, le relief naturel des objects, et
la comparaison est tout a la avantage de ce genre de reproduction" (Monsieur
F. Panajou, 1899).
A brief note on the History of Stereoscopy
On 21st June 1833 Mister Charles Wheatstone presented in the British Royal
Society an essay about a phenomenon until then scarcely known: the importance
of binocular vision in the perception of distance, depth and relief of objects
in Nature. Continuing his research, Mr. Wheatstone published five years later,
in 1838, an essay illustrated with stereoscopic hand-drawings. It was the
start of one of the most strange (and little known nowadays) photographic
techniques.
Traditionally, Stereoscopy is divided in Stereoscopic Drawing (Stereodrawing)
and Stereoscopic Photography (Stereophotography or Stereography). The present
essay deals with the later technique. The words are of Greek Etymology:
Shortly after the first public presentation of the Daguerreotype of Monsieur
Louis Jacques Mande Daguerre, in August 1839, a number of photographers began
experimenting the possibility of using cameras for making stereoscopic views.
The first extant stereograph was made by Mister Wheatstone himself in 1841.
He wrote that some stereographs had been made by Mister William Henry Fox
Talbot in the late 1830's, but sadly they are now lost. Mister Talbot did not
use the Daguerreotype, but his own Calotype, the first negative-positive
process in the History of Photography.
In 1849 Sir David Brewster perfected the optical stereoscopic device for
viewing stereographs, giving a great encouragement to those who cultivated
this photographic technique, which in few years became enormously popular and
began producing thousands of stereographs of landscapes, monuments, or famous
people. About 1860 it was very common in the high class homes of Europe and
North America to possess a stereoscope in the hall or studio, for enjoyment
of visitors.
Herr Hermann Helmholtz performed in 1857 the first scientific research on
Stereoscopic Optics, doing the mathematical calculations on focal length,
convergence of axes and stereo base for the correct perception of the
phenomenon. It became clear that Stereoscopy is based on an optical illusion
that only happens in the brain of the observer, with the observer seeing
simultaneously with the two eyes, each eye (or each image seen by each eye)
from a slightly different angle. An image seen with a single eye, or an image
drawn or photographed without using stereoscopic techniques, receives the
name of Monoscopy: Monoscopic Drawing (Monodrawing) and Monoscopic Photography
(Monophotography). Stereoscopy is also called tri-dimensional image, and
Monoscopy bi-dimensional image.
In 1907 the Autochromatic Lumiere Plates were released to the photographic
market. They were of panchromatic spectral sensitivity, developable by the
reverse process to direct positive made public by Signore Rodolfo Namias in
1898. The process of Messieurs Louis Lumiere and August Lumiere gave colour
images by additive trichromy. The first colour stereographs were thus made.
Stereoscopy also was applied to Cinematography since the start of the XX
Century, having a great importance in the decennium of the 1950's. There were
also some efforts for making Stereoscopic Television and for Auto-Stereoscopy
(this one used mainly for decorating walls or memorial books, and for tourist
post cards), but since 1970 Stereoscopy began to fall into oblivion, in spite
of the efforts made by camera and stereoscope factories for its revival.
Nowadays Stereoscopy is mainly used for civil or military Aerial Photographic
Survey, and for other scientifical or technical purposes.
There are in the World some associations devoted to Stereoscopy. Those
photographers interested in the subject may consult the list of names and
active hyper links after the main body of this essay. Some of them sell books
and stereoscopic materials.
Methods for making Stereoscopic Photography
There are in existence four main methods for making Stereoscopic Photography,
each one of them with possible variations. The main ones are:
Stereoscopic camera
Using a stereoscopic camera, with two optical objectives of parallel axes and
a single camera body. It has the advantage of making possible to photograph
moving scenes, but the two disadvantages that for any scenes (moving or not)
the stereo base is fixed and the axes are parallel. It is possible to modify
the stereo base and to converge the axes when photographing unmoving scenes,
simply by using the stereoscopic camera as if it were a monoscopic camera
shooting twice (see point four, below).
Beam splitter
Using a beam splitter, which can be adapted to a monoscopic camera. It
splits the image from the scene into two stereogrammes, located side by side
inside the space of a single monoscopic photogramme. It has the advantage of
photographing moving scenes, like in the prior case, with also the same
disadvantages of fixed stereo base and of parallel axes, plus the added
disadvantage of producing stereogrammes of half size. It is also possible to
use the beam splitter for unmoving scenes as if it were a monoscopic camera
shooting twice, although it is more difficult to do it with a beam splitter
than with a real stereoscopic camera.
Two monoscopic cameras shooting simultaneously
Using two monoscopic cameras of identical format and optical system, with
the same pointing attitude and regulation of adjustments (focus, relative
opening, exposure time, et cetera) and shooting simultaneously. It is the
best solution, because it makes possible to modify stereo base as well as
convergence, but it has the serious difficulty of needing an exact
synchronisation between the two cameras when photographing moving scenes.
For unmoving scenes, the synchronisation between the two cameras is not
necessary.
One monoscopic camera shooting twice
Using a single monoscopic camera, shooting twice. The stereo base can in
this case be modified or by moving the camera sideward or by rotating the
subject to be photographed, between the two shots that will form the
stereocopic image. This is the process that will be explained in the present
essay. It makes possible to modify stereo base as well as convergence of axes
at will, although it also has the very serious drawback of only making
possible to photograph unmoving scenes. In most cases the stereoscopist
photographer who choose this method must avoid a casual portrait, because his
characters will appear somewhat moved in the ten or twenty seconds that are
necessary for moving the camera sideward, pointing again, and shooting the
second time. Only with models well trained for complete immobility, like
statues, is possible the use of this method. You may try with hardened French
legionnaires or with poker face British policemen, because it is a matter of
fact that with most mortals this method does not work at all.
Method when using a single monoscopic camera shooting twice
Viewers
Viewers: get two small hand-held viewers for observing diapositives by
transmitted light, in the format 135 of 24 X 36 millimetres. Viewers that are
placed on a table or that are held at a distance from the eyes and have their
own inner light are absolutely useless. You must get very simple viewers, of
those which are approached to the eyes (either with their own inner light or
-more commonly-, which are pointed to any external source of strong light).
They must be identical. A pair of them costs less than five pounds. The focal
length of these viewers is invariably 50 millimetres, which makes convenient
to use in the camera objective lenses of the same or similar focal length in
order to record correctly the stereoscopic image.
Camera
Camera: use preferently a camera with manual controls, format 135 of 24 X 36
millimetres. Any other format could be used, but there would be the problem
of viewing the stereographs, because viewers in other formats are difficult
or impossible to obtain in the market, and perhaps would have to be made
under special order or by the photographer himself, while viewers for the 135
format of 24 X 36 millimetres can be easily bought. Photographers using an
automatic camera must give priority to the relative opening of the diaphragm.
Objective lens
Objective lens (optical system): use an objective of normal focal length,
which for the mentioned format is more or less between 45 and 60 millimetres,
and ideally between 50 and 52 millimetres. It is possible to use other focal
lengths, although for a correct reproduction of the stereoscopic effect it
requires a compensation of the stereoscopic base according to mathematical
formulae of some complexity. An approximation to solve the calculation will
be given later in this essay. More precise results may be obtained by other,
more complex equations. Using either too long or too short focal lengths
without compensation, especially the short ones, will provoke distortion and
change in the apparent size, distance and stereoscopic depth of the elements
in the scene. Those aberrations, when well studied and partly compensated,
will produce a strange vision of the World that is exclusive of Stereoscopy.
The biggest relative opening of the diaphragm (f/2, f/1.4 or bigger) has no
particular importance, except when wishing a short depth of field in order to
discriminate the nearest plane by leaving the other planes out of focus,
which is something that in Stereoscopy must be done rationally, so as not to
lose the points of visual reference in the texture of the farthest plane,
this is to say, of the background.
Film and developing
Film and developing: use a film for getting a direct positive image in a
single developing process, this is, a film intended for diapositive slides
(transparencies). It would also be possible to convert a negative into a
positive by contact, but it would be a complexity and there would always
exist a loss of quality. If having only films intended for negatives, then it
is better to develop them by reversal process to direct positive. The above
mentioned Namias process is the best one for this purpose.
Monochromy: most films accept reversal development to direct positive. Kodak
Kodalith Ortho, Technical Pan, Infrared High Speed, Tri-X, Plus-X, T-Max,
Ilford F-50 Plus, SFX, Delta, FP-4, HP-5, Agfa Pan, Scala, Fuji Neo Pan,
Forte Pan, Foma Pan... all of them yield good results, but the exact process
for reversal development varies from one emulsion to another. Films of low
sensitivity are better, and also high contrast developers based on
Hydroquinone and Caustic Hydroxide: Kodak D-8, D-11, D-19, D-72, D-85,
Dektol, Kodalith A+B, Tetenal Dokulith, Dokumol... are some examples.
Photographers who compose their own developers enjoy greater control on
contrast, density of blacks and grain size, than what is possible with any
commercial ready made developers. That is because small grain is a necessity
in Stereoscopic Photography, but development in depth is a necessity in
reversal development, and the only way to comply with these two apparently
opposed conditions is by using an emulsion of low sensitivity, combined with
a developer of high energy and adding a restraining agent: Potassium Bromide,
Benzotriazol or Nitrobenzimidazol are the most common ones.
If possible avoid films of high sensitivity or developers of low contrast.
Agfa Scala is the only film in the world especially made for yielding in the
reversion a monochromatic positive image of the highest quality. There is
also some self reversal duplication film for Cinematography: it yields a
positive image when developed in a process for negatives. Another alternative
is using chromogenic film Ilford XP-2 or Kodak T-400 CN and developing it in
Kodak E-6 process for trichromy and polichromy, but the resulting positive
image will not be true black and white, it will be cyan and white. All the
mentioned films suffer a considerable loss of sensitivity when developed to
direct positive (except Agfa Scala and the films for duplication, in which
the nominal sensitivity has already been compensated). For instance, the
chromogenic films XP-2 or T-400 CN whose sensitivity in the C-41 process for
negatives is of 400 ASA 27 DIN, must be exposed at an approximate index of 70
ASA 19-20 DIN when wishing to develop in normal E-6 process for positives.
That means a loss of about two and a half steps, and it is approximately like
that with all emulsions.
Trichromy and polichromy: There are in the World about a dozen films for the
already referred E-6 process for positives. Most of these films are made of
three sensitive layers (trichromy), one or two are made of four layers
(polichromy, or more precisely, tetrachromy). None of two layers (bichromy)
is being made any longer. They are balanced either for day light or for
tungsten light (wolfram). All colour films nowadays marketed in Photography
are based in the substractive colour process, while about the additive colour
process it can be said that it became almost exclusively restricted to
photomechanical processes for graphic printing, where tetrachromy
(polichromy), using four inks, is still a common process. In all cases of
development to direct positive, be it monochromy or tri-polichromy, exposure
is preferably calculated for the high lights, and in case of doubt it is
better to sub-expose or to sub-develop.
That is exactly the opposite to what is done for developing into negative, in
which exposure is calculated preferently for the shadows and in case of doubt
super-exposure or super-development is the rule. The reason for the difference
is that it is much easier to decrease by chemical means the high density of a
dark image (even at the cost of losing detail in the high lights), than to
increase the low density of a light image. Exposure latitude for diapositives
is much narrower than for negatives, because in the negative-positive process
it is possible to correct exposure errors during the positivation, inside
certain limits, but exposure errors cannot easily be corrected in the direct
reversal positive. In spite of that, strongly traditional photographers like
Mister P. A. Stonemann, CSS Dixieland, do not use photometres (light metres),
but only a posographic table of personal construction (a list of typical
light conditions and their values in Lux), and apply mathematical formulae
for determining all values that are determinable in Photography.
Scene
Scene: using a single monoscopic camera and therefore needing to make two
records of the same scene from different angles, choose a scene having all
elements still, or with slow or distant movements, or movements axial to the
line of vision of the camera, so that in for instance thirty seconds it be
not possible to detect important movement. Near leaves moved by slight wind,
distant leaves moved by strong wind, slow or distant animals coming towards
the camera or receding from it, following the axial line of vision, near
rivers of slow waters, distant clouds or smoke moved by strong wind, all
those are examples of scenes that can be photographed, in spite of not being
completely still. It all depends on the speed, distance and direction of
movement -relative whither the camera be pointing-, and of the time needed by
the photographer for exposing the first stereogramme, moving the camera
sidewards, pointing again, and exposing the second stereogramme, according to
what will be explained later.
Near plane
Near plane: choose a main motive of pictorical interest, and be careful in
avoiding the visualisation by the camera of any other element at shorter
distance than the main motive of interest. If no angle of pointing could make
possible the avoidance of those too near elements, then try to leave them at
one side of the image composition, or then out of focus by opening diaphragm
or by using an objective lens of longer focal length (in this last case a
compensation of the stereoscopic base must be done). If any of those
solutions prove to be impossible, then the nearest element in sharp view will
be named as "near plane", and it will command the calculations coming later.
The ideal goal is to have the main motive of interest also as the near plane.
In Stereoscopy it yields good results a detailed and planned work, which
includes looking for the best angle whence to make both photographs of the
stereographic pair, without undesirable elements appearing in one or the other
of the stereogrammes, or in both.
Far plane
Far (distant) plane: the visible background being apparently behind the main
motive of interest (and in reality perhaps much farther), is named "far
plane" or "distant plane". When seen by the camera, that distant background
seems to surround all or the most part of the main motive. The farther the
distance of the background will be the better, provided that it be clearly
visible. It is also better to choose an angle of pointing that place the main
motive against a far plane of well defined texture, for making easier the
visualisation of the stereoscopic effect. A heterogeneus background of trees,
clouds or mountains is better than a homogeneus background of grey sky or of
sea flat like a looking glass. It does not matter if there exist in the scene
other planes even farther. With regard to our calculations, the "far plane"
is that one seeming to be behind the most part of the main motive.
Focus
Focus: focus the near plane when wishing a short depth of field , even if
provoking a slight out of focus in the main motive. Notwithstanding, the
desirable procedure would be to have the main motive also as the near plane.
It is possible to focus the main motive when wishing a longer depth of field,
but only if the near plane be still inside a good focus, because the
important thing to remember is that the near plane must always be in
excellent focus, except when being a small or lateral element just incidental
to the scene. A calculation of circle of confusion diameters at a minimum and
a maximum distance may be necessary for precision work.
Long depths of field are commonly used in Stereoscopy, but there are some
situations in which for one reason or another it may be preferable to
photograph with a short depth of field: for clearly distinguishing certain
elements from their background, or for having a great resolution of details
in a plane of special interest, even at the cost of leaving the other planes
out of focus. Optical resolution of tiny details will diminish a little when
enlarging depth of field, by closing diaphragm aperture or by using an
objective lens of longer focal length and of similar quality. Regarding the
far plane it does not matter if it be slightly out of focus, or even if it be
considerably out of focus, always provided that some of its texture be visible
enough for having a visual reference of the stereoscopic effect. A completely
flat background will diminish the perception of stereoscopic depth.
Relative opening and exposure time
Relative opening and exposure time: as it has now been said, a big opening
and a short time will give a little more of resolution of tiny details in the
focused plane, at the cost of shorter depth of field for the other planes,
which would therefore be out of focus, but in Stereo Photography it is
generally preferred a long depth of field for having with good focus at least
the nearest plane and the main motive, and perhaps also some focus in the
farthest plane behind the main motive. This is the place for saying some
words about chromatic sensitivity and general sensitivity of photographic
emulsions.
By the Grottus-Draper Law, only actinic radiation will provoke a chemical
reaction of development in the emulsion, this is, only the part of the
electromagnetic spectrum to which the film or plate be sensitive will have an
impact in the form of latent image, therefore only that part of the spectrum
will be developed into a visible image. Using films of the ultra magenta
(ultra violet), achromatic (blue sensitive) or orthochromatic (isochromatic)
types, remember the low actinity possessed by colours of long wave length and
low vibration frequency, when these colours be predominant in the scene (like
for instance with tungsten light -incandescence lamps-), and increase
proportionally the necessary exposure, respect to that which would be
calculated for panchromatic films.
With infra red films it should in theory be the opposite: to decrease exposure
before tungsten lit scenes, due to the high amount of infra red radiation
possessed by incandescent lamps. But infra red Photography (approximately
beyond 700 nanometres) is generally made with filtres that absorb a part of
the visible radiation (Kodak Wratten 87 A, visually red), or the whole of the
visible radiation (Kodak Wratten 89 C, visually black), and in these cases
the exposure must be increased according to the factor of filtre. The "visual
focus" does not exactly coincide with the "chemical focus" when working with
non panchromatic emulsions. The image is formed in a different plane from the
plane that a human photographer perceives as the correct focus. Therefore the
focused distance must always be corrected, no matter which colour may be
predominant in the environment.
Another problem is the Schwarzschild Effect of Reciprocity Failure, which
substantially affects films of high sensitivity. With those of low
sensitivity, however, this effect is not so pronounced, and the necessary
increase when using very long or very short exposure times (or when using
very small or very big relative openings) is comparatively minor. It comes to
be almost zero for the Lippmann Emulsion used in scientific Photography
-particularly in Astronomy-. However, with most films an increase of exposure
will be necessary, calculated in multiplicative factors based on an
exponential progression that varies according to the film and to its
development. This is particularly important when the low intensity of
environmentally available light force to exposures that may last for some
minutes, some hours, or even some days for each photograph -or for each
single stereograph-, or else when working with Fresnel zone plates or with
stenopeic cameras ("pin hole" cameras), which usually possess extremely small
openings.
Convergence of axes
Convergence of axes: When a stereograph be made with cameras in parallel
axes, the vertical margins located at both sides of the combined image show
no stereoscopic effect, because each margin has been recorded on only one of
the stereogrammes, and not on the two stereogrammes that form the
stereographic pair.
For preventing that, the axes of the cameras must converge on an easily
distinguishable point located in the near plane, or located in the main
motive if this motive be not too separated from the near plane. However, the
convergence of axes must not be complete when photographing motives located
too near to the cameras, because in that case it would produce the
"convergence error", in which a scene element of square form would appear in
the photograph with some form of parallelogramme, or rather, of trunk of a
rectangle triangle lying horizontally. The left stereogramme would show the
left side as bigger and the right stereogramme would show the right side as
bigger, which would provoke a difficulty of visualisation. The cause of this
phenomenon resides in the laws of perspective, and it cannot be completely
corrected. As a safe working rule, the distance between the point of
convergence in the scene and the cameras must not be smaller than the focal
length of each camera multiplied by one hundred:
Convergence distance => focal length x 100.
For cameras of the 135 format with a focal length of 50 millimetres, it would
be safe to converge the axes to a minimum of five metres between cameras and
point of convergence.
The distance of convergence could be shorter than that, when the scene lack
elements with well defined straight lines, in which case it would still be
acceptable a minimum distance of convergence equal to the focal length
multiplied by fifty:
Convergence distance => focal length x 50.
In the example given, of the 135 format and 50 millimetres of focal length,
convergence could reach a minimum of two and a half metres. For precise
photographic works, distances should always be measured from the immmergence
nodal point of the cameras' optical objectives, and not from the plane onto
which the real image be projected. The exact location of the immergence and
emergence nodal points in complex optical systems can only be determined by
mathematical calculations, which we shall not explain here. For more general
works, measuring from the image projection plane as opposed to measuring from
the nodal point would represent so small a difference, that no pains need to
be taken in determining the location of the nodal point. Measurement can
acceptably be done, as an approximation, from the centre of the image plane.
This consideration will be valid throughout this essay for most of the
formulae given, when measurement of distances be a necessary factor.
When it be imperative to photograph a motive located too close to the
cameras, then it will also be necessary to converge the axes onto it, and
therefore a compensation for a smaller stereoscopic base should be done, or
else the mentioned convergence error would be unacceptable. We shall not
enter now in the complexity of trigonometrical calculations for the correct
convergence angle. At the moment it will be enough to say that in such cases
the two cameras' axes could be pointing, as a safe working procedure, to some
intermediate angle between the complete convergence (onto the nearest plane
or onto the main motive) and the parallelism of axes pointing to the
infinite. Using the camera's range finder or the polished glass, rotate the
camera vertical or horizontally until having the chosen point of convergence
in the centre, or in one of the four corners of the image (or in any other
part of the image, but determine as precisely as possible which part it is,
because the same framing operation will have to be done for recording the
other stereogramme, in the second shot).
Near plane and far plane distances
Near plane and far plane distances: Measure as best as possible the distances
in straight line from the cameras to the near plane and from the cameras to
the far plane. Remember what has been said about measuring from the
immergence nodal point for precision works, or from the centre of the image
plane for more general works. The measurements must take as reference, at the
other end, the centre of the most characteristic element located in the near
plane, and the element in the far plane apparently located behind the most
characteristic feature of the main motive. The distance of the main motive
itself does not matter much, only an estimation must be done that the main
motive be not much farther than the near plane, and even better, that the
main motive be also the near plane. If photographing, for instance, the tower
of a mediaeval castle from a levelled ground, know Ye that the distance from
the cameras to the tower's base is one of the two cathets of the rectangle
triangle, the tower's height (to the top or to another chosen point) is the
other cathet, and the distance from the cameras to the tower's top (or to its
chosen point) is the hypotenuse. Knowing the values of the cathets, apply the
Pythagoras theorem to calculate the hypotenuse:
(horizontal cathet^2 + vertical cathet^2) v2 = hypotenuse
Example: horizontal cathet (distance from cameras to tower's base) twenty
metres, vertical cathet (tower's height) ten metres. Therefore:
(20^2 + 10^2) v2 = (400 + 100) v2 = 500 v2 = 22.360 679 774 9
little more than twenty two point thirty six metres is the hypotenuse
(distance from cameras to tower's top or to its chosen point).
The expression "^2" means "raised to the power of two", and the expression
"v2" means "extracted to the root of two". For example, four raised to the
power of two equals sixteen: 4^2 = 16, and its inverse operation is sixteen
extracted to the root of two, which equals four: 16 v2 = 4
If using a theodolite, a sextant, or another goniometric device, it is also
possible to calculate it from the sinus of the angle formed by the camera's
vertical inclination relative to a levelled ground. It has no need of being a
precise measurement, Stereoscopy allows for an ample margin of tolerance when
not used for technical ends. The distance to the near plane, however, must be
measured more rigourously than the distance to the far plane. When the far
plane be at a great distance relative to the near plane, then the far plane
could be considered at the infinite.
Stereoscopic base: when using a single camera shooting twice, stereoscopic
base is the lateral distance separating the location of the camera at the
first shot, from its location at the second shot, giving slightly different
views of the scene. Stereoscopic base could also be defined as the angle of
rotation of the main motive between the first shot and the second shot (in
some cases it is preferable to keep the camera immobile and rotate the motive
instead). Using other procedures, stereoscopic base is the lateral distance
separating the locations of the two cameras shooting simultaneously, or else
the two optical objectives of a stereoscopic camera, or else the two mirrors
of a stereoscopic beam splitter.
When the objectives of the two cameras (or of the single camera shooting
twice, from different locations) rather than being parallel, happen to be
converging onto any point of the scene, then the stereoscopic base will not
be exactly a straight line, it will be an arc of circumference with centre
in the point of convergence, and could be measured in radian or in any other
angular unit. However, it could also be measured in metres or in any other
length unit the internal string of the secant that connects the two extremes
of the arc of circumference. We shall use this last measuring method in the
present essay.
For simplicity, let us suppose that the focal lengths of the camera (or
cameras), and of the viewers that will act as stereoscope, are all of them
identical, let us say 50 millimetres.
Let us further suppose that we photograph with a near plane at a distance of
more than five metres, and therefore we can quite well keep the camera (or
cameras) axes in parallel without loosing much stereo effect in the main
image. We have no need to converge axes.
We shall be confronted with one of two cases: either the far plane being so
distant that it could be considered as being at the infinite (in which case
we shall only need to measure the distance to the near plane and use a very
simple formula), or else the far plane being comparatively close to the
cameras in relation to the near plane (and therefore we have the need of
measuring the distances to both planes and of applying a little more complex
formula, derived from the previous one).
First case, far plane at infinite:
Distance from cameras to near plane / 50 = stereoscopic base
Formula where the number 50 was statistically obtained, after examining the
stereoscopic perception in many human individuals and calculating average
values. The stereoscopic effect that results by applying this formula
multiplies by a factor of about two hundred the minimum stereoscopic
perception, in binocular vision, considered normal for that species. The
value of stereoscopic perception varies from an individual to another,
existing some individuals who are capable of detecting a slight difference
between the distances of two different planes, in fact in planes that for
most observers would seem to be located at the same distance. On the other
hand, there are also individuals who almost totally lack stereoscopic
perception: they can see with their two eyes, but their brain does not
interpret the combined image in terms of depth. They possess a binocular but
a monoscopic vision, same as there are other individuals who are daltonic
and do not perceive colours.
The average values of stereoscopic perception also vary from a race to
another and from a species to another. The values given here are calculated
for the White Race (Leucodermata) of Homo sapiens sapiens. Other specialists
in Stereoscopic Photography suggest the number 30 in the same formulae where
here we are suggesting the number 50, not only because they may be at least
partly based on other statistical studies, but also because of their personal
preference of provoking a rather exaggerated stereoscopic effect. An
intelligent stereoscopist photographer must research by himself, and not
accept with dogmatic faith what is being said in the present essay or what
may be said by any other author. This advice is valid for every field of
knowledge. The reader must think with critical intelligence, and then decide
what he should do.
But, just as an initial experimentation, it may be reasonable to begin with
the value of 50 given here, with the value of 30 given by other authors, or
with another value in between, let us say about 40. Let us suppose that our
divisor value of 50 be provisionally accepted. Let us remark that such a
divisor must be used with parallel axes, because if photographing a very near
motive or foreground it may be necessary to work with converging axes, and in
such a case a divisor of 100 should be applied, making a smaller stereo base
as it has already been explained in the appropiate epigraph.
Let us show some examples.
First example, near plane at 50 metres from the camera and far plane at the
infinite:
50 / 50 = 1
The stereoscopic base must then be equal to one metre.
Second example, near plane at 4 metres and far plane at the infinite:
4 / 50 = 0.08
Here the stereo base must be 0.08 metres, or 80 millimetres.
We may have the case that the far plane be not "at the infinite" (meaning, so
distant that it be unnecessary to measure its distance), but at a finite
distance (meaning, that it be near enough to justify the measuring of its
distance). Then we must apply a derived formula, which if using parallel
axes will be:
((Far plane X Near plane) / (Far plane - Near plane)) / 50
Resulting in the correct stereoscopic base.
Third example, near plane at 50 metres and far plane at 150 metres from the
camera:
((150 X 50) / (150 - 50)) / 50 = (7500 / 100) / 50 = 75 / 50 = 1.5
The stereoscopic base must be equal to 1.5 metres, one metre and a half.
Observe that it was one metre when the far plane was at the infinite.
Fourth example, near plane at 4 metres and far plane at 6 metres:
((6 X 4) / (6 - 4)) / 50 = (24 / 2) / 50 = 12 / 50 = 0.24
Ergo the stereo base must here be 0.24 metres, or 240 millimetres. Observe
that it was 80 millimetres when the far plane was at the infinite, but now
the base has become threefold.
As it has been said, the divisor 50 must be applied when photographing with
axes in parallel. If photographing at close range and using converging axes,
then the divisor must be 100.
The result calculated as appropiate stereoscopic base for a scene with an
"infinite" far plane could be multiplied by as much as five with a finite far
plane, when this far plane be very close to the near plane. When photographing
individuals against a wall, for instance, or when photographing coins or other
nearly flat objects against a dark surface, or in Aerial Photography of the
ground from a flying machine. The stereo base must not be exceeded much more
than that, because in that case there would not be a frontal binocular vision,
but two images representing each of the opposing sides of the main motive or
scene, and for most observers it would be impossible to fuse the two images
into one single combined image. They would see a double image, not a stereo
image.
Another necessary condition is to have the two images absolutely identical,
except in the angle from which they have breen recorded. Focal length,
relative opening, exposure time, intensity, colour and direction of light,
focusing, film and its development, framing, synchromism of the two shots (or
immobility of the scene), everything must be as equal as possible in the two
stereogrammes. To be sure, in the stereo base as well as in the other factors
there is always some tolerance, in some of them more than in others, but for
making perfect Stereoscopy it is necessary to control all the relevant
factors with the maximum of precision reasonably possible.
Viewing
Viewing: Once the stereographs have been exposed and developed, simply mount
separately each stereogramme in a slide mount, and so You will have two
stereogrammes (in the form of two slides) for each stereograph. Then
introduce each slide in the right position into a monocular viewer, having so
a pair of monocular viewers, each one of them loaded with a stereogramme of
the scene. In this combination, the two monocular viewers are now forming a
single binocular viewer, called stereoscope, and the two stereogrammes are
forming a stereograph.
It is important to take the care of having the left side image in the left
hand and the right side image in the right hand, because accidental change of
viewers to the wrong hand (which happens very often with beginners in Stereo
Photography) will provoke an "inverted Stereoscopy", also called
"anti-Stereoscopy" or "pseudo-Stereoscopy", in which the far elements of the
scene seem to be near and the near elements seem to be far. This is a most
strange vision of the World, particularly because sometimes the element that
appears as nearer to the observer happens to be partly occulted by another
element that appears as farther, a phenomenon that is an incomprehensible
absurd, and which invariably astonishes and even fascinates those who observe
it for the first time.
Approach the two viewers to Your eyes, not too near, and quietly rotate them
in various directions until the two images that You are observing be fused in
a single combined image, then keep the two viewers static in that position.
At that instant You will enter into the magic world of Stereoscopy !
Resume of operations, step by step
That is enough as an introduction to the subject. There are many documents
about Stereoscopy published in the World Wide Web of computers, but it is
necessary to know something on the subject for being able to discriminate
those with accurate information from those with misleading fantasies. Anyway,
most of those documents have no technical information at all, they are just
showing the work of an individual photographer or a group of photographers,
or they simply refer the "social activities" of some club or association,
where Stereoscopy may be the main "raison d'etre" or may be just an
incidental "hobby".
After having perused our present essay, serious and professional photographers
with a strong interest in technical matters are very welcome to write to us
and ask specific questions. We shall answer their doubts to the best of our
knowledge. Electronic post address at the bottom of the page.
Short Bibliography on Stereoscopy
Principles of Stereoscopy, by H. C. Mc Kay
Researches in Binocular Vision, by Kenneth N. Ogle
Stereo Photography, by Fritz G. Waack
Stereoscopic Phenomena of Light and Sight, by Theodore Brown
Stereoscopic Photography, by Arthur W. Judge
Stereoscopie, par E. Colardeu
Hyper links
Stereoscopic Society
National Stereoscopic Association
Stereoscopy
Stereo Camera Club
Gesellschaft für Stereoskopie
Vereniging voor Stereofotografie
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Stereos = Volume or Space, Skopos = Image, Photos = Light, Graphos = Record
"Image in volume recorded by light".
The expression "x" means "multiply" and the expression "/" means "divide".
Information and resources on Stereoscopy
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Information and resources on Stereoscopy
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