Free viewing of stereo pairs refers to viewing two photos, drawings, etc. to see a 3D effect without using a viewing device to assist the eyes in "fusing" the two images together. Two basic methods may be used:
Pairs of stereo images for parallel viewing have the left image placed on the left side, and the right image on the right. To view, "defocus" your eyes in a manner similar to day-dreaming. After a time you will begin to see "double" images. If you begin to concentrate on the images in the center of your view, you should be able to get them to merge into a single 3D image.
Pairs of stereo images intended for cross-eyed viewing have the image intended for the left eye on the right side, and the right eye on the left side. To view a picture in stereo, display it in its entirety and cross your eyes so that the two halves of the image merge. It is often helpful to over-cross the eyes and let them relax somewhat until the scenes merge into a stereo view. Concentrate your attention on some prominent feature of the scene, preferably in the distance, and get these to merge first. Once full merging takes place you will be able to move your eyes around the scene at will. Don't be discouraged if you don't achieve the stereo effect easily. Some people have more trouble than others, but cross-viewing is a skill that nearly everyone can learn.
If you are new to the technique, don't overdo it. Crossing the eyes tires muscles that are not used to being treated that way. Take frequent breaks until you have built up muscle strength through this unusual exercise. (You will not hurt your eyes in the process.)
If you have trouble viewing an image cross-eyed, it may help to use a pencil or other object held a short distance in front of the stereo pair. By focusing on this, one can get the eyes to cross at approximately the right angle.
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