From times ti
times I get a question or some remark. Often the person
had not success in his/her research because the results
are so dependent on the key words. There are also questions
only about this website. These are the topics of this page..
Anyone must
be aware that there are many ways to practice stereoscopy,
either in photographic or cinematographic views. There's
no denying that new technologies bring newer solutions.
To us to well
understand the principles, not to wander in unfunded or
unsuitable concepts
Sometimes it
is good to immerse again in the fundamentals
A nice
3D view 3D is not the image
with the depth of relief pressed to extremes but the image
which renders and suggests
to the best the shooted
scene.
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Actually,
6 questions-answers:
Are there
methods other than the stereoscopic ratio to SR to evaluate
the degree of relief?
In which book
can we find the method of the stereoscopic ratio?
Why the 1/30
rule is not given in these pages?
Why the zoom
factor M is smaller for tele when the subject is greater in
the picture?
Why images
identical in light and color are necessary ?
Why
the pictures shall be perfectly sharp with a large depth of
field?
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Are there methods other than the
stereoscopic ratio to SR to evaluate the degree of relief?
My first
researches in the 90's being unsuccessfull, I thought that
the evaluation of degree of relief shall not be so complicated
and it would be faster to try to set myself some way to
do it.
Today looking
at the opposite issues I'am quite satisfied to have done
it. These issues are probably not all because I'm far examining
all potential sources.
-
- For the evaluation of these other
methods the SR is calculated with the parameters given by
their authors. As a comparison scale:
- with k = 0,0004, B = 65
mm, for a subject at D = 4 m
- - on background
at L = 4m backwards it is SR = 20
- - on background
at L = infinity it is SR = 40.
- Above 50 the 3D ratio SR is considered
as excessive and be applied to exceptional situations only.
-
-
- As summary:
-
The methods
set on both distances between camera- subject and subject-background
are the only rational ones.
The method
described opposite are there because they are set on the
3D geometry, applying the angular parallax and the depth
between plans in perspective. The quantification of only
the limits of the effect of relief is not a sufficient evaluation.
.
Both concepts
stereoptic power Ps and proximity R have similarities and
the same pros and cons. The draw of curves Ps R as per D
and L show the same shape, identical to SR. The difference
is a scale factor, which was foreseeable looking at the
similarities of the formulas.
-
-
- As conclusion:
-
The stereoscopic
ration is set only on the eye perception, its keeness and
the angular parallaxes of the stereoscopic images, which
makes it the most rational and universal method evaluating
plainly the achieved 3D effect. It applies the physical
basics expressed by L. Cazes, it is factual and relevant.
Note that
the stereoscopic ratio including the eye keeness and angle
parallaxe is directly linked to the resolution, to the pixels
and to the linear parallaxe.
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- PS. Thanks for advising any other
existing method..
-
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1910, Emmanuel Colardeau, stereoptic
power
Partially published in 1910 in the monthly
bulletin of the Stereo Club Français and in library
in 1923, E. Colardeau continued the L. Cazes' theory including
the simple 1/50 rule. The 3D effect vanishes in the increase
of the subject distance and the nearness of the background.
E.Colardeau expressed it by the inverse of the length of
the neutral zone which he named the stereoptic power Ps.
Badly he reduced the formula to the case of the background
at infinity and deduced Ps = 1 / 0,005 D² or
200 / D² knowing that 0,005 is the approximate value
of k/B. take again this formula with our corresponding notations
will be B = Ps . k . D² but will only applies
when the background is at infinity. Thus the Ps values for
distances of 1,8m, 3,3 m, 6 m et 162 m are respectively
50, 15, 4,5, 1.
1926, Arthur W. JUDGE, the proportionality
of relief
Leaning onto the hyper-stereoscopy
which he held when wisely used without exaggeration for
obtaining a better relief rendering, A. W Judge expressed
the proportionality between the relief effect and the base,
and between the distance of the subject. When we take
a stereoscopic photograph with a lens separation equal to
n times that of the eyes, the stereoscopic effect will be
that of an object I/n the size seen at I/n
the distance. He haven't pursued and given any formula.
This double proportionality would give the following relation:
En/Eo = Bn/Bo = Do/ Dn = p ou En
= p.Eo.(Bn/Bo).(Do/Dn)
A. W. Judge took up again most
of E. Colardeau's datas and the 1/50 rule but ignored the
Ps. Unfortunately he didn't considered the distance of the
background.
1984, Jean Mallard, 1990 Olivier
Cahen: the proximity
In «L’image en Relief»
re-published in 2011, O. Cahen takes again the J. Mallard's
method issued in the bulletin n°677 of the S.C.F. Set
on the proximity R defined as the inverse of a distance,
the formula is R = 1/D1 – 1/
D2 where D1
et D2 are the distances of two plans
to the camera. The calculated value is given a 1/1000 of
the unit and is thus the milliertem. With our notations,
distance D and depth L, the formula is R = 1/ D – 1/ (D+L)
or R = L / [ D (D+L)] similar to expressions in the general
formula of Colardeau, or the SR formula without the factors
like s.B/k.
The proximity R doesn't give the
degree of 3D depending on other parameters (base,
angular field, etc). those factors are added by ratios (B/Bo,
etc). The reference values (Bo, etc) and the method are
not well described. O. Cahen finalized it. That way it is
completed but makes the method uneasy to understand and
not simple to use. The acceptable 3D values are loose. The
comparison milliertems and SR gives 400 milliertems
~ SR50 and 500 milliertems ~ SR80
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In which book can we find the method
of the stereoscopic ratio?
- In fact, no book yet, the whole
of the method is in this web site.
The section
stereoscopy of this site started in January 2007 with clearly
the page about SR. In addition to the table sheets there
is a calculator easier for daily use. .
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I have set the principles of the
stereoscopic ratio in 1997. A paper was published first
in the review Aerial Eye in 1998 (volume 4.3 pages
8, 9, 18). Then a presentation was done in 2000 at the KAPiCa
conference in California.
- It has been published in the bulletin n° 956
of the Stéréo-Club Français, December
2012 and more in n° 957, January 2013.
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Why the 1/30 rule is not shown in
these pages?
The answer
is why should it be?
By who and
how this rule called 1/30 has been set? Until now I haven't
found formerly its exact origin and neither found any technical
setting thoroughly justifying it .It could follow an advise
for binocular stereoscopic apparatus with fixed base. Later
it has been declared "rule of the thumb" however
I more consider it as guesswork and unreliable
Thanks for
advise about historic documents on this topic if any.
This rule
produce many image distorsions and several stereoscopists
recommend other value or restrict its use.
ISU, the
International Stereoscopic Union recommend as "golden
rule" Dn ~ B x F*
Dn
distance to nearest plan; B e base;
F*
equivalent focus length in 24x36.
The three
variables are necessarily in millimeters.
This simple
rule gives acceptable results varying as per the distance
between the nearest plan and its background because it's
not taken in account. The calculated SR range from 30 to
62 as maximum..
Knowing that
it is fully advisable, not the 1/30.Sachant cela, elle est
recommandable, pas le 1/30..
You can use the calculator for determining the SR with the datas of
any rules and compare. the results..
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A priori it determined the distance
to the nearest subject using binocular cameras "Shoot
with the first plan further 30 times the base".
The other version is the maximum disparity shall not
be more than 1/30 of the width of the picture. This
has given the value for 35 mm films 36/30 = 1,2 mm. However,
being in first for portraits framed vertically, it should
have better been 24/30 = 0,8 mm. Then applied to the landscape
format 36 / 0,8 = 45, almost the 1/50 Cazes recommandation..
As it is the 1/30 considering a
65 mm base and the first plan at 6m produce an excessive
relief as soon as the background is at more than 3 m behind
the subject.
Then that rule has been reversed
and lead astray for base calculation. Thus with a first
plan at 6m the and the SR when the background is far is
reaching 83. When the small size stereograms were viewed
through stereoscopes, the excessive relief was weakened
by the restitution method. The frst plan could be at the
nearest with a far background.
Nowadays make the 1/30 the utmost
rule by some, with limitations by others is no more appropriate.
Is it not better to forget it and understand what makes
the relief and how it is given back.
The rule 1/50ème set
by L. Cazes, taken again by E. Colardeau, A. W. Judge and
many others, funded on angular parallax is more suitable
and issued by a technical argument with respect of natural
stereoscopic vision.
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Why the zoom factor M is smaller
for tele when the subject is greater in the picture?
- This comes from the reasoning.
With a tele lens the subject is seen bigger because the
angle of field is smaller. M is the variation of the horizontal
field angle A, not the focal length. It is more universal
with reference to the field angle of the eye.
-
- Note M = 2 . tan (A/2) =
37 / F* with reference to 35mm film
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Because the argument is on the
angle of field it is the range factor of this angle which
has been hold.
For a same effect of relief the
base is increased when wide angle is set and lessen with
tele lens.
It is an optimized adjustment of
the base more necessary for landscape views depending when
they are shot with wide angle or tele lenses, or for not
be over the limits when there is a very near first plan.
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Why images identical in light and
color are necessary ?
It is giving more visual
comfort but it is not an absolute requirement. Apply it or not
is rather a matter of preference or of opinion. In some cases
a difference is more beneficial, but this also is not a hard-and-fast
notice. |
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Normally the more images will be
different in light, colors, contrast, etc,... the more it
will be tiring to our eyes.
Personnally I set with a slight
difference the twin cameras because I feel that this small
difference let increase the dynamic performance of the picture,
like some automatic HDR It is more valuable IMO because
it seems to me that when some parts are too much white in
some parts or too dark in others for both images, the image
is harder to perceive than in 2 D. However, a slight
difference will let a better rendering both in very light
and very dark tones, in warmer and cooler colors.
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Why the pictures shall be perfectly
sharp with a large depth of field?
sharp/blurred
image crossed view
There again, it is
more a matter of appreciation than of scientific argument. As
in any photographic picture it is essential that the subject
is sharp. Whatever the background be sharp or blurry will provide
the same effect of relief except on an even background. As in
2D photography a subject in front of a fuzzy background will
be more worthy.
NB Because of that
the methods of calculation of the base established on the values
of the depth of field and the sharpness of the picture are funded
on a false assumption. |
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If that was true how to explain why we can
perceive as good in 3D and absolutely sharp a pair stereoscopic
pictures which one is sharp and the other is loose.
When there is a too broad depth
of field which is impossible to produce even with a small
aperture I use this subterfuge. O one image focus is done
on the former plans and on the othezr on the last ones.
During the restitution, when our
eyes look on the former plans they are all perceived as
sharp, and the same for the furthest ones. All is due to
the genius of our brain.
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