visual axis
Also called line of sight, visual line, fixation axis
The line from the point of fixation through the nodal point of the eye to the foveola; in single binocular vision the visual axes of the two eyes meet at the point of fixation1,2. It is a line of the schematic eye, and the line that can be measured in clinic is the line of sight, from the fixation point through the center of the entrance pupil3.
It is easily confused with the pupillary axis, which passes through the center of the pupil at right angles to the cornea. The angle between the two is angle kappa, normally 3 to 6 degrees and positive because the fovea lies temporal to the back pole of the eye3.
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 kappa3. 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; 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 aligned1.
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