Stereoscopy

Stereoscopy is the science and art that deals with the use of binocular vision for the observation of overlapping photographs or other perspective views and the method by which such views are produced.  Essentially most of us with ìnormalî eyesight have stereoscopic vision (i.e. The ability to see and appreciate depth of field through the perception of parallax.)

Monocular Depth Perception

There are clues to the depth of field of objects which can be perceived/appreciated monocularily. Monocular or one eye depth perception deals with in-born cues that humans instinctively employ when viewing objects. These are things we do we donít normally think about. Such cues or clues to the distance we think an object is at include the fact that close objects typically appear:
  1. Larger;
  2. Brighter;
  3. To be seen in more detail; and,
  4. To obscure the view of objects that are more distant.
There are limits, however, to monocular depth perception. Put a coin on end on a table and view it with only one eye. You see only the edge of the coin. The same can be said for looking straight on at other objects. A chimney viewed from directly above can appear to be as an o. Is it a well; or is it a smokestack?

Binocular Vision

With binocular vision each eye sees a different picture and the brain fuses the two images into one. {Talk about the dominant eye, and experiment with glasses that turn the world up-side-down.} Stereoscopic vision is not an all or nothing affair. Individuals, depending on a number of factors (e.g. interpupillary distance, near or far sightedness, etc.) have stereoscopic vision to varying degrees. It is best if interpreters who do a lot of work with stereo have nearly equal vision in both eyes. Don't worry if you don't have equal eye strength.  Very few interpreters work only with stereo, or with stereo all the time. The people who do this type of work are the photogrammetric engineers who work all day on stereo plotting equipment.

Depth Perception

The normal interpupillary distance in humans is 2.5 to 2.6 inches. If we could increase this distance we would increase our perception of depth. Stereo pairs greatly stretch this normal eye base (interpupillary distance) and give up the exaggerated 3-D photographic effect we perceive when viewing the stereo pairs.
 
Figure 8-1. Stereo Eyes
 

 
 
 
 

Figure 8-2. Stereo Photos

Now, normally when viewing objects greater than 1,500 feet it 2,000 feet away the special ability to perceive depth is essentially lost.  Look, say at a far off line of trees on a flat plane. It is very difficult to tell if they are really in a line or staggered. Basically what your eye brain mechanism will do in this case is employ other clues to help determine the depth of the trees (again, these are essentially the monocular cues described above). With normal eyes from an aerial view we would see a tree like this; while with a stereo pair it would appear like this.

From this you can begin to see the difference between unaided stereo viewing and viewing objects and features in the environment with the aid of stereo pairs.

In normal unaided stereo the lines of sight converge, and it is this convergence that gives us depth perception. Basically, then when our eyes focus on a near by object they also converge so that the lines of sight from each eye intersect at the object. However, in stereo photo vision our lines of sight are essentially parallel or focused at infinity. Now, because of a process called accommodation we have a problem. Accommodation relates to the change in focus of the eye with distance. Basically, convergence and accommodation go together. As the eye focuses on an object they also turn (or move) so that the lines of sight intersect at the object. The issue (or
problem if you will) with stereo vision is that to ìget stereoî we must maintain parallel lines of sight while focusing our eyes at close range. This is not a normal situation and may cause strain on the eyes.
 
 

Figure 8-3. Sausage Exercise

The "Sausage Exercise" can be helpful in developing the ability to see stereo. Essentially in this exercise you focus your eyes on a distant object and then slowly bring your forefingers into the line of vision. The farther apart your fingers and the larger the sausage when it forms the more nearly parallel are your lines of sight.

Proper Use of Stereoscopes

The following are some guidelines that will help you use your stereoscopes properly. They are important and should be kept in mind when performing stereo based interpretations:

Be sure that the photos are properly aligned, preferably with the shadows toward the viewer. (Having the shadows away from the viewer can cause terrain reversal or "false stereo".)

  1. Be sure to keep the eye base and the long axis of the stereoscope parallel to the flight line.
  2. Try to maintain an even glare free illumination of the images and make yourself comfortable.
  3. Keep the lenses of your stereoscope clean.
  4. Do Not Attempt Stereoviewing For Long Periods in the Beginning.

Problems/Issues Involved in Viewing Stereo

  1. People with eyes of unequal strength may have difficulty seeing stereo. If you wear glasses it is best that they be worn when stereo viewing.
  2. Poor photographic illumination, misalaigned prints or uncomfortable viewing positions may result in eye fatigue.
  3. Illness or severe emotional distress may create sensations of dizziness when using strereoscopes.
  4. Pseudo stereo can be caused by:
    1. Erroneous reversal of prints;
    2. Viewing photos with shadows falling away rather than towards the interpreter.
  5. Objects which change position from one photo to another cannot be viewed in stereo.
  6. In areas of high vertical relief, scale differences in adjacent photos may make it difficult to obtain a 3-D effect.
  7. Dark shadows or clouds may prohibit stereoscopic study by obscuring details of the scene on one print or the other.
  8. Individuals who have difficulty with stereoscopic viewing, should not attempt unaided eye stereo viewing.