The primary responsibility of the eye is to focus light. Light rays must focus precisely on the retina in order for your vision to be clear. At the front of your eye, the cornea provides the bulk of the eye’s focusing power. Internally, the lens inside your eye fine tunes the light, contributing to your ability to read.
The most common vision problems are refractive errors, more commonly known as nearsightedness, farsightedness, astigmatism and presbyopia. Refractive errors occur when the shape of the eye prevents light from focusing directly on the retina.
The cornea and lens bend (refract) incoming light rays so they focus precisely on the retina at the back of the eye.
What is refraction?
Refraction is the bending of light as it passes through one object to another. Vision occurs when light rays are bent (refracted) as they pass through the cornea and the lens. The light is then focused on the retina. The retina converts the light-rays into messages that are sent through the optic nerve to the brain. The brain interprets these messages into the images we see.
What are the different types of refractive errors?
The most common types of refractive errors are nearsightedness, farsightedness, astigmatism and presbyopia.
Nearsightedness (also called myopia) is a condition where objects up close appear clearly, while objects far away appear blurry. With nearsightedness, light comes to focus in front of the retina instead of on the retina.
Farsightedness (also called hypertrophy) is a common type of refractive error where distant objects may be seen more clearly than objects that are near. However, people experience farsightedness differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant farsightedness, vision can be blurry for objects at any distance, near or far. Learn more about farsightedness.
Astigmatism is a condition in which the eye does not focus light evenly onto the retina, the light-sensitive tissue at the back of the eye. This can cause images to appear blurry and stretched out. Learn more about astigmatism.
Presbyopia is an age-related condition in which the ability to focus up close becomes more difficult. As the eye ages, the lens can no longer change shape enough to allow the eye to focus close objects clearly. Learn more about presbyopia.
Who is at risk for refractive errors?
Presbyopia affects most adults over age 35. Other refractive errors can affect both children and adults. Individuals that have parents with certain refractive errors may be more likely to get one or more refractive errors
What are the signs and symptoms of refractive errors?
Blurred vision is the most common symptom of refractive errors. Other symptoms may include:
- Double vision
- Glare or halos around bright lights
- Eye strain
How are refractive errors diagnosed?
An eye care professional can diagnose refractive errors during a comprehensive dilated eye examination. People with a refractive error often visit their eye care professional with complaints of visual discomfort or blurred vision. However, some people don’t know they aren’t seeing as clearly as they could.
How are refractive errors corrected?
Refractive errors can be corrected with eyeglasses, contact lenses, or surgery.
Understanding Your Prescription
Common vision problems include myopia, hyperopia, astigmatism, and presbyopia. The degree of these vision problems are measured in units called diopters. A diopter is used to describe the light bending properties of an optical system. Glasses that correct for nearsightedness are expressed as a negative (-) number and as a positive (+) number for farsightedness. The more nearsighted, farsighted or astigmatic you are, the higher your prescription in diopter.
Your prescription is composed of three numbers:
For example,-5.00 -1.50 x 180 represents a typical prescription to correct a common vision problem.
- First Number: -5.00 identifies your degree of nearsightedness or farsightedness. The sign identifies whether you are nearsighted (- sign) or farsighted (+ sign).
- Second Number: -1.50 identifies your degree of astigmatism. The number IS written either with a + sign or a – sign.
- Third Number: 180 identifies the axis, which indicates the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.
Therefore, a prescription of -5.00 -1.50 x 180 indicates that the patient is moderately nearsighted, with a moderate degree of astigmatism in a horizontal direction.