Visual acuity
Measure of the eye's ability to resolve fine detail.
Visual acuity (VA) is a measure of the spatial resolution of the visual processing system, commonly referring to the clarity of vision and the ability to recognize small details with precision. It depends on both optical factors (such as the shape of the eye and cornea) and neural factors (including the retina, neural pathways, and brain). Visual acuity is typically measured by identifying optotypes on a chart from a set distance, with 6/6 (20/20) vision considered a reference value for normal performance.
- field
- Vision science, ophthalmology, optometry
- known_for
- Standard measure of spatial resolution of vision, including distance and near acuity
- measurement_units
- Snellen fraction (e.g., 6/6), decimal acuity (e.g., 1.0), visual angle (arc minutes)
- normal_reference
- 6/6 (20/20) vision, corresponding to resolving 1 arc minute gap
- key_conditions
- Myopia, hyperopia, astigmatism, amblyopia, macular degeneration
Lore & Background
Visual acuity is most commonly assessed as distance acuity, using charts such as the Snellen chart or Landolt C chart, where optotypes are presented at a standard distance (e.g., 6 metres or 20 feet). The fraction 6/6 indicates that a person can discern details at 6 metres that a standard eye would see at the same distance; 6/12 indicates half the spatial resolution. Acuity can also be expressed as a decimal (e.g., 1.0 for 6/6) or in terms of visual angle, with the critical gap for a 6/6 optotype being 1 arc minute.
Reader's Guide
Visual acuity is a fundamental metric in eye care, used to diagnose refractive errors such as myopia and hyperopia, and to assess neural conditions like amblyopia or macular degeneration. While optical corrections (eyeglasses, contact lenses, refractive surgery) can address many refractive errors, neural factors may limit acuity even after correction. Acuity is highest in central (foveal) vision and declines steeply toward the periphery, following an inverse-linear function. Importantly, acuity alone does not capture overall visual function, as it only measures resolution of small details in the center of the visual field. The measurement is standardized by norms such as EN ISO 8596, and normal individuals often achieve 6/4 or better acuity depending on age.
Did You Know?
- Visual acuity of 6/6 corresponds to resolving a gap of 1 arc minute, which is 1/5 the size of a standard optotype.
- Acuity in peripheral vision declines according to the formula E2/(E2+E), where E is eccentricity in degrees and E2 is about 2 degrees.
- The minimum angular resolution of the human eye is 28 arc seconds, or about 0.47 arc minutes.
- Normal individuals often have acuity of 6/4 or better, depending on age and other factors.
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