visual axis
Also called line of sight, visual line, fixation axis
The visual axis is the line from the point of fixation through the nodal point of the eye to the foveola, and in single binocular vision the visual axes of the two eyes meet at the point of fixation.1,2
The visual axis belongs to the schematic eye, while the line a clinician can measure is the line of sight, which runs from the fixation point through the center of the entrance pupil.3
A second line, the pupillary axis, passes through the center of the pupil at right angles to the cornea, and the two are easily confused. The angle between the visual axis and the pupillary axis is angle kappa, normally 3 to 6 degrees and positive because the fovea lies temporal to the back pole of the eye.3
More detail
Strictly, the angle measured in clinic is angle lambda, between the line of sight and the pupillary axis, although it is usually called kappa.3 The corneal light reflex sits where the line of sight crosses the cornea, slightly nasal to the center of the pupil in a normal eye, and the Hirschberg test reads this displacement. A large positive angle, for example when the macula has been dragged temporally, makes the eyes look turned out while the cover test shows they are aligned.1
Sources (3)
- Kanski, Clinical Ophthalmology (2024), ch. 18
- Pickwell's Binocular Vision Anomalies, glossary
- Adler's Physiology of the Eye, ch. 4
Reviewed October 2026