Amount of aniseikonia compatible with fine binocular vision: some old and new concepts.

Random Dot Stereograms (RDS) are useful, but are sometimes misleading in assessing the degree of binocular cooperation in the presence of aniseikonia. RDS may give positive results up to 15% aniseikonia, whereas a fine central binocularity is not possible with more than 5% image-size difference. It is suggested that stereopsis may not be the only parameter to test; stress is given to the importance of peripheral motor fusion in maintaining the eye alignment.