Stereoacuity was also measured using the TNO, Frisby, and Titmus stereoacuity tests. To measure the threshold of depth discrimination, subjects were required by means of a hand control to align two electronically controlled spheres at viewing distances of 1, 3, and 6m. Subjects with normal binocular function, reduced binocular function, and apparently absent binocularity were included. A child-friendly variable distance stereoacuity test (VDS) was developed, with a method for determining the binocular depth threshold from the combined monocular and binocular threshold of depth of discrimination (CT). In such circumstances, a combined monocular and binocular threshold of depth discrimination may be measured-stereoacuity conventionally referring to the situation where binocular disparity giving rise to retinal disparity is the only cue present. Although the cue of binocular disparity underpins stereoacuity tests, there may be variable amounts of other binocular and monocular cues inherent in a stereoacuity test. Measurement of stereoacuity at varying distances, by real or simulated depth stereoacuity tests, is helpful in the evaluation of patients with binocular imbalance or strabismus.