The distance between the person's eyes and the testing chart is set so as to approximate ' optical infinity' in the way the lens attempts to focus (far acuity), or at a defined reading distance (near acuity).Ī reference value above which visual acuity is considered normal is called 6/6 vision, the USC equivalent of which is 20/20 vision: At 6 metres or 20 feet, a human eye with that performance is able to separate contours that are approximately 1.75 mm apart. Optotypes are represented as black symbols against a white background (i.e. VA, as it is sometimes referred to by optical professionals, is tested by requiring the person whose vision is being tested to identify so-called optotypes – stylized letters, Landolt rings, pediatric symbols, symbols for the illiterate, standardized Cyrillic letters in the Golovin–Sivtsev table, or other patterns – on a printed chart (or some other means) from a set viewing distance. Visual acuity is a measure of the spatial resolution of the visual processing system. Definition Eye examination for visual acuity Visual acuity alone thus cannot determine the overall quality of visual function. Visual acuity is a measure of how well small details are resolved in the very center of the visual field it therefore does not indicate how larger patterns are recognized. At 2 degrees eccentricity, for example, acuity is half the foveal value. The decline is according to E 2/( E 2+ E), where E is eccentricity in degrees visual angle, and E 2 is a constant of approximately 2 degrees. the decline follows approximately a hyperbola). Acuity declines towards the periphery first steeply and then more gradually, in an inverse-linear fashion (i.e. However, acuity in peripheral vision can be of equal importance in everyday life. as a measure of central (or foveal) vision, for the reason that it is highest in the very center. Visual acuity is typically measured while fixating, i.e. When optical factors are corrected for, acuity can be considered a measure of neural functioning. Examples of conditions affecting the brain include amblyopia (caused by the visual brain not having developed properly in early childhood) and by brain damage, such as from traumatic brain injury or stroke. Examples of conditions affecting the retina include detached retina and macular degeneration. Neural factors that limit acuity are located in the retina, in the pathways to the brain, or in the brain. For example, in the case of myopia, the correction is to reduce the power of the eye's refraction by a so-called minus lens. Refractive errors can mostly be corrected by optical means (such as eyeglasses, contact lenses, and refractive surgery). Other optical causes of low visual acuity include astigmatism, in which contours of a particular orientation are blurred, and more complex corneal irregularities.

Normal refractive power is referred to as emmetropia. A similar poorly focused retinal image happens when the combined refractive power of the cornea and lens is too low for the length of the eye except that the focused image is behind the retina, yielding hyperopia. When the combined refractive power of the cornea and lens is too high for the length of the eye, the retinal image will be in focus in front of the retina and out of focus on the retina, yielding myopia. Causes of refractive errors include aberrations in the shape of the eye or the cornea, and reduced ability of the lens to focus light. This ability is compromised in people with hyperopia, also known as long-sightedness or far-sightedness.Ī common optical cause of low visual acuity is refractive error (ametropia): errors in how the light is refracted in the eye. Another visual acuity is near acuity, which describes someone's ability to recognize small details at a near distance. This ability is compromised in people with myopia, also known as short-sightedness or near-sightedness. The most commonly referred-to visual acuity is distance acuity or far acuity (e.g., '20/20 vision'), which describes someone's ability to recognize small details at a far distance. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain. Optical factors of the eye influence the sharpness of an image on its retina. Visual acuity depends on optical and neural factors.

Visual acuity ( VA) commonly refers to the clarity of vision, but technically rates an animal's ability to recognize small details with precision. A typical Snellen chart that is frequently used for visual far acuity testing.

Pupil size chart normal