Visual Acuity

Visual acuity is the sharpness of vision: the ability to distinguish fine detail, usually using central vision. The cornea and lens focus light onto the retina, and the macula—especially the fovea—converts that focused image into the highest-resolution visual information. Blur can therefore arise from a refractive problem, such as uncorrected myopia, or from disease affecting the lens, retina, optic nerve, or visual pathways. A Snellen fraction describes performance at a fixed distance. The numerator is the testing distance; the denominator is the distance at which a person with standard vision can read that same line. Thus, 20/20 at 20 feet is normal distance acuity, equivalent to 6/6 at 6 metres. A result of 20/40 means the client must be 20 feet away to read what standard vision can read at 40 feet. The result is below normal, but the evidence-based definition of impaired acuity used here is worse than 20/40—20/45 or poorer. Acuity is only one part of a visual assessment. It does not measure peripheral visual fields, colour discrimination, pupil responses, ocular pressure, eye alignment, or the health of...