stereoacuity
Also called stereo acuity, stereoscopic acuity, stereothreshold, stereopsis threshold
The smallest horizontal difference between the two eyes' images, called retinal disparity, that a person can detect as depth. It is measured in seconds of arc, a lower number is better, and stereopsis is the sense of depth itself3,4. An object slightly in front of or behind the fixation point falls on slightly different retinal points in the two eyes, and the cortex reads that disparity as depth1,3.
In clinic a threshold of 15 to 30 seconds is excellent and 60 seconds is a common rule of thumb for normal1,2. Random-dot tests prove that both foveae are fusing, because their figures cannot be seen with one eye; contour tests such as the Titmus fly can be passed with monocular cues or with anomalous correspondence2.
More detail
Stereoacuity is a hyperacuity, finer than the resolution of letters, and under laboratory conditions it can reach 2 to 7 seconds of arc1,3. Stereopsis is not the same as fusion: eyes can fuse without fine stereopsis, as in monofixation syndrome, where only gross stereopsis remains5. Stereoacuity worse than about 40 to 60 seconds is the usual trigger for the Worth four dot test and the 4 prism diopter base-out test.
Sources (5)
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 2
- Kanski, Clinical Ophthalmology (2024), ch. 18
- Steinman, Visual Perception: A Clinical Orientation, ch. 10
- Eperjesi and Rundström, Practical Binocular Vision Assessment, ch. 7
- Pickwell's Binocular Vision Anomalies, glossary
Reviewed October 2026