Many years ago (when Germany was still split in
two parts) I got an optical experiments set from
"the other side". This set was a real treasure
chest, among other things you could create a
stereo slide viewer from two single slide viewers.
And there was one stereo slide of an amazing
quality that you could watch with this.
Some years later I managed to buy a "Viewmaster", a small
device to watch very small slides that are arranged on
a disc of paper. The disc is rotated so that you can
always see a pair of stereo slides through the lenses.
This is also one of the very old (and quite famous)
examples of stereo photography.
I had the idea that it should be relatively easy to
create such stereo slides, well, at least they are just two
photos that were taken with two cameras mounted in the
distance between the human eyes. After a short search
in the internet I knew two more things:
Here I will present the common techniques,
publish some do-it-yourself descriptions and
give you some practical hints on how to create good
- I was not the first to have this idea.
- There are many more possible solutions to
create stereo images, each of them in its own
range of costs and quality.
The three dimensional impression that we get when
we look into a landscape arises from the evaluation of
information from both of the images that we see with
our two eyes. The brain evaluates each single images
as well as the differences between them. Even from having
a look at one single picture we can gather information
about the depth. The following hints are evaluated by
the brain to extract this information:
The density (or the size) of the single objects on the
picutre. With an increasing distance from our point of view
the objects look smaller. Also the distances between to
far away objects seem to be smaller. This leads to the
impression that far away areas are far more dense and
compressed. The image to the right show this effect.
Parallaxis of Movement:
If you move your head then the images of nearby objects
move faster over your retina than those of far away
objects. With a growing distance the movement seems to
slow down (for example if you look out of your driving
car). The evaluation of images that have been taken
after each other can provide information about the
distance to the shown objects. The picture to the right
shows two objects that are shifted in front of the
eyes. The angle between the both positions of the
nearby object is much bigger than the angle between
the far away one though the distance the objects have
been shifted is the same.
The differences between both single pictures provide an
essential part of the depth information of what we see.
The distance between the human eyes is usually about
6.5cm. If we watch some object we always see two
images from a slightly different point of view.
Each of them is a two dimensional image of the
environment so from each single picture we cannot
create a complete three dimensional representation.
But if you compare both single images you can
conclude some depth information from the differences
between both of them. The principle is the same as
with the parallaxis of movement but it is not
necessary to move the head because we already have
two images with a different viewpoint.
The upper image shows that the distance to both objects
can be derived from the different viewing angles
in the single images.
The second figure shows that a single image is not enough
to do this. Different distances to some objects can
produce the same viewing angle to a human eye (in this example
the left one). Only the other eye provides the missing
information that is necessary to estimate the distance
When viewing a single picture the parallaxis of movement
cannot be used to extract information of depth because the
two dimensional image does not change when we move our
head (holograms are special in this regard). The texture
gradient provides us even in this case with some information
about the depth of the scenery. But obviously this is
not enough to create the impression of a threedimensional
scene. Only the additional information of a second
image provides us with a realistic impression of depth.
But how can you get two slightly shifted images of the
same scene? There are many practical methods to
achieve this. These have very different levels of
limitations and costs so I will discuss some of them
Using one camera, shooting two images in a sequence:
The most simple way is of course to use an existing
camera, shoot the first picture, shift the camera by
a few centimeters and shoot the second one. The limitations
of this method are easy to see:
When shifting the camera
you have to be carefull to
do preserve the viewing direction of the lenses. If there
is a slight turn in the direction you get what we are used
to if we squint: If we turn each of our eyes more to the outer
rim then both of the images do not overlap. We see two
different images that are not being correlated by our
brain. If we look cross-eyed our brain refuses to correlate
the different images too. In both cases we tend to
get a bad headache after a few moments. The same will
happen if you watch stereo images that were taken
with a non-parallel viewing direction.
Obviously there should be no
movement of objects
between both of the shots. Such differences are very
disturbing for the three dimensional image (because
our brain uses exactly this kind of differences
between the images to derive the information of depth).
This kind of differences between two images destroy
our impression of depth.
So this is only the easiest and cheapest
solution to create
stereo images. You can use this for a short test, but it's
not suitable for taking serious photos.
Using one camera and a beam splitter:
For normal camera lenses there exist some prism devices called
beam splitter wich fit into the filter thread.
The splitter has two lenses from wich the entering light
is splitted so that the light from the right lens falls
onto the right half of the film, the light from the left
is lead to the left half of the image. That way the
camera takes a stereo image that has only half of the
width of a normal image. To view this kind of image
correctly a viewer is necessary that ensures that each
eye only gets to see its own half. These viewers
exist for slides as well as for paper prints.
With a beam splitter it is easy to take stereo pictures
even from moving objects. The devices have a strictly
parallel viewing direction for each half so there is
no problem with this either. A disadvantage is the
format of the images: It's only half as wide as a
normal photo, so it's a quite narrow upright
format. There exist no equipment for the projection
of slides that were taken using this technology.
Using two cameras:
If you own two cameras you can attach them next to each
other on a bar. If the cameras are small it is even
possible to mount them with the distance of the
human eyes (6.5cm) from lens to lens. If the cameras
have zoom lenses it can be difficult to make sure they
both show the same detail of the scene. Even fix focus
lenses of the same type do not always show exactly
the same clipping. Often even lenses of the same type
differ in the brightness of the produced images
(this can be adjusted using the setting of the
film sensitivity or using some correction setting
for the aperture).
The remaining main problem is the synchronous
release of the cameras. There exist cable releases
for two cameras that can do the job. Much more precise
is the use of electronic synchronisation mechanisms if
the release of the cameras is electric. I use a pair of
Olympus XA2 cameras that I enhanced by leading the
electric contacts of the release to the outside of
the camera and connecting them to a small box with
a common release for both cameras. The necessary circuit
can easily be made by yourself. Here is a
description for building a synchronous release.
You can even mount both cameras on a bar so that both of the
bottom sides point to each other. That way you create a
stereo camera for upright format photos.
In this case you have of course to keep to the distance of the
human eyes too. At the moment my stereo camera uses three
small cameras from wich I can always choose two for the
actual picture. So I can choose between upright and
landscape format. The following figures show the
Using two (or more) synchronised cameras is the
most flexible solution. If you choose taking slides
you can watch them using just two normal viewers
(one for each eye). For projection of the stereo slides
you need two projectors, two polarization filters
and a pair of glasses with polarization filters.
The quite small costs of small rangefinder cameras
with fix focus lenses (especially if you buy them
second hand) enable the construction of a cheap
and universal stereo camera.
Here I will proceed...
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