The last step
of the restoration process, would it be through slide, on
print, or on screen, depends on the restoration backing
and possible viewing equipment. This combination modifies
in its turn the perception of the image, and therefore the
stereoscopic effect.
The question
is now:
Which
stereoscopic effect will be obtained at the end?
I named apparent stereoscopic ratio SR^ the ratio which is felt at
the end of the restoration.
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Methodology:
It is still
to evaluate the stereoscopic effect, and to compare it to
the initial stereoscopic effect SR.
The combination
of the restoration backing and the visualization system
compel to compare the circle of confusion of the one or
the other, and to take for the calculation of the stereoscopic
effect the most restricting of both. To that are added the
restrictions caused by the successive losses of resolution.
Examples will help to see
the processus and to lay figures.
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- Relations:
- The reduced relation is t = SR.a.F.P
- Les restriction conditions on SR are:
SR >
c' / (a F)
SR >
c'' / (a.F.P) or SR
> c''' / (a.F.P)
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The circle of confusion of backing
is c''
The circle of confusion of viewing
is c'''
the stereoscopic ratio is:
SR^ = t / c'''
if c'' < c'''
SR^ = t / c''
if c'' > c'''
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- Viewing slide:
In this case, without enlargement,
P=1
t = u = SR . a . F
Viewer with ocular focal length
78mm to the distance H=65mm of the slide.
Apparent stereoscopic ratio:
SR^ = SR . a . F / c'' =
SR'' = SR'
Thus, SR^ =
0,47 SR
with SR
> 2,1
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Circle of confusion of image on
backing:
c'' = c' = 0.03mm
Circle of confusion of viewing:
c''' = a.RH/100 = 0.02mm
Thus c'' > c''' and SR^ = t
/ c''
Remark:
The stereoscopic
restoration on slides with viewer is good, and excellent
with a quality viewer. Add it the full frame, the best contrast,
the best intrinsic resolution.
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- Viewing on photographic print:
-
- by naked eye:
Observe to naked eye corresponds
to H=250mm (conventional distance) and G = 1
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Example of a stereoscopic pair
shot with a 35mm film camera and a lens focal length 35mm,
viewed on two prints 10x15cm by eye-cross viewing.
Enlargement ratio is P=5
Apparent stereoscopic ratio:
SR^ = SR . c . F . P / c'''
=
SRx 0,0004x35x5/0,25
Thus, SR^ =
0,28 SR
with SR
> 3.6
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Circle of confusion of image on
backing:
c'' = c' . P = 0,15mm
Circle of confusion of viewing:
c''' = 0.25mm
So c'' < c''' and
SR^ = t / c'''
The enlargement ratio is P = 5
because the laboratories print by cutting large borders,
fie our cared frames!
Remark:
The stereoscopic
restoration on print viewed by naked eye gives an acceptable
result which is easy to achieve.
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- with a stereoscopic viewer:
Example of a stereoscopic pair
shot with a 35mm film and a 35mm lens, viewed on two prints
6x9cm with a viewer focal length R=134mm to the distance
H=110mm.
Apparent stereoscopic ratio:
SR^ = SR . a . F . P / c''
=
SRx 0,0004x35x3/0,09
Thus, SR^ =
0,47 SR
with SR
> 2.1
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The enlargement ratio is 90/36
= 2,5 but with a "laboratory" printing work, P=3
is more suitable.
Circle of confusion of image on
backing:
c'' = c' . P = 0,09mm
Circle of confusion of viewing:
c''' = a.H.R/100 = 0.015mm
So c'' > c''' and
SR^ = t / c''
Remark:
The stereoscopic
restoration on print and viewer gives a good theoretical
result..
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- Viewing on computer screen:
Example of a stereoscopic pair
which each view is 320x240 pixels for a crossed-eye viewing
with naked eye. The image has been shot with a digital camera,
sensor 1/1,8" length = 7,2mm of 4 Mpx (2272x1704) and
7,3mm lens.
Each view is cropped to 320x240
pixels to be viewed as stereoscopic pair.
Therefore, it is X=0,36x320=115mm
Apparent stereoscopic ratio:
SR^ = SR . a . F . P / c''
=
SRx 0,0004x7,3x16/0,65
Thus, SR^ =
0,07 SR
with SR
> 14
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The enlargement ratio is:
P=X/U=115/7,2=16
Circle of confusion of image
on backing:
c'' = 0,65mm
Circle of confusion of viewing:
c''' = 0.25mm
So c'' > c''' and
SR^ = t / c''
Remark:
The stereoscopic
restoration of a digital image on computer screen is far
to have the restoration richness of the other processes.
This is obviously caused by the low resolution of the screen.
We have to remind that the low 3D ratio fields disappear
and that a photograph all with 3D nuances risks to
wash out.
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