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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