Aniseikonia is a term I coined for a difference in size of the optical images of the two eyes. By the optical image is meant not the retinal image but that which reaches consciousness as a perception. This definition leaves unanswered the question of where the cause of the difference is to be found, whether in the refraction mechanism or in histologic differences in the sensory elements of the retina or in the structures still higher up.
Aniseikonia throws a burden on what I am fond of calling "the neuromuscular mechanism for binocular vision." In some way the eyes must compensate for the aniseikonia, and in ordinary cases they do, so that binocular vision goes on satisfactorily. However, the amplitude of adjustment in compensation for aniseikonia is limited. Compare this amplitude of adjustment with other amplitudes of adjustment, viz., the amplitude of accommodation which compensates for certain common errors
WALTER B. LANCASTER. ANISEIKONIA. Arch Ophthalmol. 1938;20(6):907–912. doi:10.1001/archopht.1938.00850240021001