Visual acuity

Visual acuity is the ability of the visual system to distinguish fine detail, commonly measured by resolving separate points or identifying progressively smaller symbols.

Minimum angle of resolution: The smallest angular separation at which two points can be distinguished as separate. It defines the resolving threshold measured by many visual-acuity tests.
Snellen chart: An eye chart with rows of letters in progressively smaller sizes, used to measure distance vision. It is the classic method for reporting distance visual acuity.
Visual field: The area of space visible to an eye when its gaze remains fixed. Field extent measures where vision reaches, whereas acuity measures detail resolution.
Hermann Snellen: A Dutch ophthalmologist who introduced the Snellen eye chart in the nineteenth century. His chart established a widely adopted way to quantify distance acuity.
Retinal ganglion cell: A retina neuron that transmits visual signals from retinal circuits toward the brain. Its receptive fields help determine how finely the retina samples an image.
LogMAR chart: A standardized eye chart whose letter sizes follow a logarithmic scale of visual angle. Its consistent letter spacing and scoring support precise acuity measurement.
Contrast sensitivity: The ability to detect differences in luminance or color across spatial patterns. A person can have normal chart acuity yet struggle with low-contrast patterns.
Franciscus Donders: A Dutch physician who helped establish ophthalmology and studied accommodation and refractive errors. His work shaped the clinical context in which standardized acuity testing developed.
Photoreceptor: A light-sensitive retinal cell that converts photons into electrical signals. The spacing and properties of these cells constrain fine-detail detection.
Tumbling E chart: An eye chart using rotated capital Es so patients can indicate the direction of the open ends. It measures acuity without requiring letter recognition or literacy.

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