Many visually impaired children have only one functioning eye and thus have no binocular functions. Children with two functioning eyes may be binocular (i.e. use both eyes together to form a single image of the object = fusion of the two pictures), or they may use the eyes alternately, or may only be able to use one eye, the other eye being lazy. How the eyes are used can be important when deciding on the best choice of low vision devices. For example, a child who uses his/her eyes alternately may be able to have distance correction for one eye and a near correction for the other, rather than the more costly bifocal lenses.
Stereovision is one of the binocular functions. Since the development of stereovision requires quite good alignment of the eyes and good vision during the first year of life, most visually impaired children do not have stereovision. This is sometimes thought to mean that these children do not perceive a three dimensional world but it is a misunderstanding. Our experience of three dimensionality is based on a number of visual cues, for example the relative sizes of objects, shadows, apparent movement when moving oneself, and the way near objects partially cover more distant objects. Most normally sighted persons who do not have stereovision are unaware that there is something abnormal about their vision. Similarly, the problems that visually impaired children and adults may have in the experience of spatial relationships cannot be explained by the lack of stereovision. However, loss of stereovision, after it has developed, results in poor control of hand movements for weeks or months, until new techniques for hand-eye co-ordination have been learned.
Questions related to refraction and binocularity are new to a number of educators and therapists. In order to understand the function and limitations of function of the child it is essential to know the quality of vision of the child when using each of the glasses that (s)he has, the distance at which the glasses give the clearest image and whether the child uses one or both eyes. All these details should be clearly explained by the child's ophthalmologist (if the child has been examined by an ophthalmologist) or by the specialist who has fitted the glasses.
If the educator or therapist has no help in measuring the refractive error of a child, large errors in the measurement are to be expected. This should not discourage them from trying again because even experienced refractionists may make sizeable errors when examining visually impaired children whose eyes may have unusual forms and whose fixation may be several degrees from the optical axis of the eye.
Similarly, assessment of binocular vision and which eye is favoured is often a difficult task. Simply covering the eyes in turn when the child reads may show which eye the child uses in reading or whether the two eyes are equally good for sustained near vision or distance tasks.