visual acuity

physiology

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July 14, 2026

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Visual acuity refers to the clarity or sharpness of vision. It is the ability to discern fine details and resolve two points separated in space. This is often measured by determining the smallest angle at which two lines can be distinguished as separate.12

Visual acuity is influenced by several factors, including the illumination of the target and the eye's optical system. In bright light (photopic vision), acuity is generally better than in low light (scotopic vision). The anatomical structure of the retina, specifically the density and arrangement of photoreceptor cells (cones and rods), also plays a crucial role. The fovea, a central part of the retina rich in cones, is responsible for sharpest vision.13

Standardized tests like the Snellen chart, used by eye care professionals, measure visual acuity by presenting letters of decreasing size. The results are often expressed as a fraction, such as 6/6, indicating normal vision where details subtending one minute of arc are resolvable at a distance of 6 meters.1

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perception
Effects of practice
…example, effects of practice on visual acuity were studied by requiring observers to detect simple orientation (left or right) in a row of leaning letters; e.g., . Practice tended to lower acuity thresholds, defined as the lowest intensity of illumination at which each observer could detect the orientation. Or, observers…
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…example, effects of practice on visual acuity were studied by requiring observers to detect simple orientation (left or right) in a row of leaning letters; e.g., . Practice tended to lower acuity thresholds, defined as the lowest intensity of illumination at which each observer could detect the orientation. Or, observers…
Read More

eye disease
Ophthalmological examination
…important subjective test is for visual acuity. This is usually performed by having the patient read, from a set distance, an eye chart with a series of letters of graded sizes, which become increasingly smaller as the chart is read from top to bottom. The person is asked to read…
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…important subjective test is for visual acuity. This is usually performed by having the patient read, from a set distance, an eye chart with a series of letters of graded sizes, which become increasingly smaller as the chart is read from top to bottom. The person is asked to read…
Read More