The most common reason for patients to see an eye care professional is to get a refraction (prescription) for glasses or contact lenses. The need for vision correction, with glasses or contact lenses, is NOT a disease or disability! Clear vision is determined by anatomy—the shape of the cornea, the clarity of the natural lens inside the eye, and the distance between the cornea and the retina. The cornea should be a smooth dome, and when the cornea has an irregular surface, contact lenses may be required to obtain best vision.
Snellen visual acuity is a crude measure of vision, which was developed during the Civil War. Snellen vision testing determines how well a person can see black images on a white background, at a distance of 20 feet. The ETDRS (Early Treatment Diabetic Retinopathy Study) visual acuity chart was developed as part of a study of diabetic retinopathy, in an effort to standardize Snellen vision testing. The ability to read a Snellen chart (or the ETDRS) is only one component of good vision, and people with impaired quality of vision may still be able to read all or part of the Snellen chart.
Quality of vision includes as many as 20 components of vision. Quality of vision measures include vision at all distances, vision in all lighting, vision fluctuation, binocularity (the eyes work together), smooth scanning, depth perception, color vision, contrast sensitivity, glare, distortion, multiple images, ghosting, sun sensitivity, night vision, a full visual field, and comfort. Loss in the quality of vision can substantially impact the quality of life, even in the presence of good Snellen visual acuity.
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