Stereo photography is a means by which 3-dimensional features such as terrain depth and the relative heights of buildings can be perceived by combining views from one or more 2-dimensional photos.

As strange as this may sound, it actually works by exactly the same principle that enables human eyesight to have depth perception.  When a pair of eyes focuses on an object, each eye views the object from a slightly different angle.  When the brain combines both of these images by automatically comparing and calculating the difference between the two perspectives, the two angles are "merged together", resulting in the ability to determine that one detail is closer to the viewer than another.

Much the same process happens when an airplane takes aerial photography.  As the plane passes over the ground, the camera takes pictures at certain intervals.  When one picture is taken, the features in the center of the photo appear in a vertical, directly "top down" perspective, while all the other features around the center appear at increasingly greater angles as they near the outer edges of the frame (as illustrated below in Figure #1).  By the time the very next photo is taken, the plane has moved forward and the new picture displays a new view with different perspectives from the one before.  The camera shots and the speed of the plane are regulated to provide a mutually overlapping portion on both photos (see Figure #2).  Within this portion (called "stereo overlap") all of the visible features, therefore, appear at different perspectives between one photo and the other.  A pair of photos with stereo overlap is called a "stereo pair" (such as the pair shown in Figures #2 & #3).

Often, a single photo of a stereo pair will be referred to as the "stereo mate" of the other in the pair.

Figure #1

This image (above) shows how the visible angles of landscape features, such as the buildings, appear to increase the farther away they are from the center of the photo.  This is highlighted in this second view of the same image (below).  Notice how the angles of the buildings (some of which are now highlighted in yellow for better clarity) appear to stretch off in the general directions of the red arrows.  The buildings closer to the edge show the greatest angle increase.

Figure #2

Figure #3

(Notice that these particular photos were taken along a north-south flight path.  The overlapping portion of the northernmost frame comprises the bottom  60% of the entire photo, while the matching portion of its stereo matethe southernmost framecomprises the top 60% of the photo.)

If these different perspectives are combined, the result is a view that looks 3-dimensional, in just the same way that the different perspectives of two eyes allow depth perception when combined.

The best method to view the stereo overlap of a stereo pair is with a device, such as a tabletop stereo viewer, that is designed to merge the landscape features when viewed through it.  However, it is possible to see the effect with a stereo pair of photos simply by crossing your eyes, as with the following example image (Figure #4).

Figure #4

This image is composed of two matching portions from a stereo pair laid side by side.  The red dot in the center of each portion has been added here as a guide for demonstration purposes.  If you begin to cross your eyes when looking at this picture, you should see that the two red dots seem to spilt into four dots.  If you can cross your eyes further, you should notice that the two middle dots of the four will move closer to one another.  Once they are lined up over top of each other, the area around them should look as though it has depth to it (despite the fact that you may also perceive the appearance of a certain amount of "double exposure").  Even without a guide such as the red dots, one need only find a distinctive feature present in both photos within the stereo overlap (such as a road, building, stream, etc.) and join them in the same manner.

Note that some people have trouble crossing their eyes far enough to perceive the stereoscopic view, and will find that this sample won't allow them to see the illusion of depth.  People with this problem will require a stereo viewer to see depth when using actual prints.

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