Measurement of visual acuity is necessary to determine the integrity of the visual system. When performed as part of the comprehensive ocular examination, visual acuity assessment can reveal refractive errors, optical disorders, and ocular disease.
Visual acuity should be evaluated both uncorrected and corrected, at distance and near, with each eye being tested independently. Binocular acuity is indicated in specific situations such as nystagmus (repetitive involuntary eye movements).
The distance acuity is typically measured at twenty feet (six meters) and most optotype charts have been calibrated to this standard. At twenty feet the rays of light from a distant object are practically parallel, and little accommodation is required. If the examination lane is shorter than twenty feet, mirrors and a reflective system are used to achieve the appropriate test distance. In some instances an optotype chart may be recalibrated for shorter testing distances.
Optometry Notebook 