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HOW TO USE A STEREO VIEWER
If you have never used a tabletop stereo viewer before, you're probably
wondering just what they do and how they are intended to be used.
First of all, a stereo viewer, such
as
the one pictured
above, is used to make a pair of aerial photographs (a
stereo
pair) appear as a single 3-dimensional view of an
area. This is important for
tasks like judging depth in the terrain and determining the relative
heights of buildings.
Note, however, that a
tabletop stereo viewer is only intended to be used with
hard copy pictures (prints or transparencies).
It is not designed to be used with
digital images on a computer screen.
In order to use a stereo viewer
correctly, you must first identify
the overlapping stereo
portions of both photos, as with the example stereo
photos shown below in Figure #1 and Figure #2. When
locating
the portion of stereo overlap on aerial photography, it is best to find
some distinctive feature somewhere near the center of one frame, and then look for that same feature
on the stereo
mate to that photo. In our example pair in Figure #1,
there
is a distinctive river running from top to bottom of both photos,
making the process of identifying the stereo portions of each frame a
fairly simple matter (Figure #2).
Figure
#1
Figure #2
Recognizing the stereo overlap
between two photos is not always so easy, depending upon the features
present in the frame, but it should become easier
with practice. One important
fact to keep in mind is
that the photo's Identification
Bar, usually running along the top edge of a photo, should be
placed facing the same direction for both frames.
Once you have located the overlapping
stereo portions on both photos, you are ready to use your tabletop
stereo viewer to look at landscape details in an illusion of
3-dimensions, by following the 3 step process outlined below.
Step 1
Place
a stereo pair of aerial photographs, one overlapping the other, on a
flat surface. Position the pair of photos so that the site you
want to view in one
photo
will line up approximately under one lens of the stereo viewer, and the
same site on the
stereo mate will
line up approximately under the
other lens.
(Notice in the picture on
the
right, the site has been marked with a blue arrow sticker on each photo
to assist in alignment. This technique can be helpful while you
become familiar with the stereo viewer.)
Step 2
Now stand the stereo
viewer on top of the two photographs so that each lens is directly
above the site in each photo. Each lens of the viewer will
focus primarily on only one
of the two photos in the pair, giving each
eye a different perspective of the site.
Step 3
If the photos are not
aligned well, looking through the viewer will give the
appearance
of
a double exposure.
To correct this, slowly move one of the
photographs
while looking through the viewer. As you move the photograph, one of
the images in the "double exposure" will move and the other will remain
still, as represented here in the simulated images below in Figures #3
&
#4.
(Note:
In the picture on the right, the blue arrows again indicate the site on
each photo. The distance between the arrows, is the same as the
distance between the lenses of the stereo viewer.)
Figure
#3
Continue to move one photograph
so
that the two images get
closer and closer, finally becoming one image.
Figure
#4
Now the stereo portions are
almost
overlapping.
Now the stereo portions are fully overlapped.
Similar to 3D glasses, this fools the
eyes into
combining the two photos into a single view. Landscape features of the
overlapping portions of the two photos will now suddenly appear to have
depth, giving an illusion of a view in 3-dimensions.
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