Visual Acuity

Visual acuity refers to acuteness or sharpness of vision. A per­son with normal visual acuity can see clearly and is able to distinguish fine details both close up and at some distance.

Errors of refraction are the most common causes of de­fects in visual acuity.Refraction refers to the ability of the eye to bend the parallel light rays coming into it so they can be focused on the retina. An error of re­fraction means that the light rays are not being refracted or bent properly and therefore are not adequately focused on the retina. A defect in the shape of the eyeball can cause a refractive error. Errors of refraction can be improved with corrective lenses.

Errors of Refraction

Figure. Diagram of normal refraction compared with myopia (nearsightedness) and hyperopia (farsightedness), which are errors of refraction that cause visual defects.

A person who is nearsighted has a condition termed myopia. The eyeball is too long from front to back, caus­ing the light rays to be brought to a focus in front of the retina. The myopic person has difficulty seeing objects at a distance and may squint and have headaches as a result of eyestrain. A corrective lens (e.g., eyeglasses, contact lenses) can correct this condition by causing the light rays to come to a focus on the retina.

A person who is farsighted has a condition known as hyperopia. The eyeball is too short from front to back, re­sulting in a different type of refractive error, in which the light rays are brought to a focus behind the retina. This person has difficulty viewing objects at a reading or work­ing distance. An individual with hyperopia may experience blurring, headaches, and eyestrain while performing up- close tasks. A corrective lens can correct this condition by causing the light rays to come to a focus on the retina.

In most people, a decrease in the elasticity of the lens of the eye begins to occur after age 40 years. This condi­tion,presbyopia, results in a decreased ability to focus clearly on close objects.

If a defect in visual acuity is detected, the patient is re­ferred to an eye specialist for further evaluation. Several types of specialist are involved in the care of the eyes. An ophthalmologist is a medical doctor who specializes in di­agnosing and treating diseases and disorders of the eye. An ophthalmologist is qualified to prescribe both oph­thalmic and systemic medications and to perform eye surgery. An optometrist is a licensed primary health care provider who has expertise in measuring visual acuity and prescribing corrective lenses for the treatment of refrac­tive errors. An optometrist is also qualified to diagnose and treat disorders and diseases of the eye and to pre­scribe ophthalmic medications. An optometrist is not a physician and therefore is not permitted to prescribe sys­temic medications or to perform eye surgery. Anopticianis a professional who interprets and fills prescriptions for eyeglasses and contact lenses.