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LSS principae
Yves Lhoumeau
Lionel Ruiz
LSS Team
How does it work ?

Beamer

Condensor

Fisheye
The master key:
a fisheye
A special photographic lens
Caracteristics:

Very short resultant focal

A standard back focal length (42-43mm)

A special distorsion of image

a circular picture, typically 24mm of diameter
Why can't we adapt directly a
fisheye to a beamer ?

Because with the original lens, the picture isn't
correctly focalized, and the size of beamer's
picture isn't compatible
Removing and replacing the original
lens of beamer: don't work either

Because the back focal length of the fisheye is too
short. A standard beamer's lens has 80mm-100mm
back focal length. Compare with the back focal
length of the fisheye.
Still not working...

Extend the back focal length of the fisheye with
a divergent lens, like a barlow lens
Then... What works ?

Beamer +

Condensor +

Diagonal flat mirror +

Fisheye
« LSS Principae »
to project an image with a fisheye...
« LSS Principae »

We must build an intermediate image, with


Correct image size: typically 24 mm in diameter
Correct incident ray light, like to take a frame, but
inverted...
« LSS Principae »

To build an intermediate image from the
beamer, we use the same process of
microphotograpy, but inverted.
We don't take a frame, we project a frame, by
putting additionnal lenses, (photographic
lenses), in front of the beamer
« LSS Principae »

This intermediate image can be deflected by a
flat mirror. Advantage: the beamer will work in a
« made for » position with no dust and heat
problems.
Technics: Fisheye specs


Fullframe format (24x36mm)
Diaphragm used to limit some optical
abberations in some cases
(typically used at F/D8 with a standard 50mm photographic
lens as condensor)

Focus unnecessary (doesn't change anything)
Technics: Condensor specs


Focal length and F/D is of
no importance BECAUSE:
Specifications change with:


The back focus length =>
change the incidence of the
rays of light
The size of the rear lens of the
condensor (to prevent cutting
image)
Diaphragm on condensor cuts
the image: don't use it !
Technics: Condensor specs: ideally


Back focus length as short as possible for max
field of view
Diameter of rear lens minimum approximatively
24mm or larger
Technics: Condensor in practice
If you're lucky... find
A 75 mm F/D 1.1 Heligon, which
is:



10-15mm back focus lens (vary
upon models)
A big rear lens
Exist in some variants: TV, XR,
etc...
Technics: Condensor in practice
With actual commercial lenses

A photographic objective, as
Canon 50 mm F/D 1.4 USM

To reduce back focal length, add a proxy lens,
+4 D to the system
Distance between optical parts


A constant: back focal length of fisheye: 42 mm
Correct distance: add back focal length of
fisheye and back focal length of condensor
Typically between 84 mm (42+42mm) and 52 mm
(42+10mm)
Adjustments with Beamer



Use DLP technology to increase contrast
Zoom parameter can change the field of view
and focus
Focusing the image can be made on the
beamer. If you can't, change the distance
between fisheye and condensor (probably too
long)
Field of view depend of:

Focal back lenght of Condensor

Beamer objective (zoom and focal lenght)

Beamer size of DLP
(typical 0,55 or 0,65'')
TIPS



Don't place the diagonal miror at intermediate
focus, if you don't want to see dust…accurately
Back focal length of the condensor can be
measured, using a beamer and a projected
picture on a piece of cardboard (as test pattern)
You can reduce the focal back length of the
photographic lens with a convergent lens in
front of condensor (typicaly F=250mm = +4D)
but it adds some distortions on the edge
TIPS

Focusing the projected picture need a VERY
precise alignement of condensor and Fisheye
otherwise there will be always a part of the
horizon blurred...
(like telescope collimation)

You can limit horizon blur and distortion with
fisheye diaphragm, but with loss of brigthness
Note: unecessery with Heligon lens
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