Refractive errors are one of the most common and correctable causes of visual impairment in the U.S. They occur when the eye cannot focus light properly on the retina. About 42% of Americans ages 12-54 are nearsighted (myopia), an increase from 25% percent in 1971. Farsightedness (hyperopia) is far less common, impacting just 5-10% of people in the U.S. Both of these are refractive errors – a person who is nearsighted can see close objects clearly, but has more difficulty seeing distant objects. Conversely, a person who is farsighted sees distant objects clearly but has problems seeing close objects. People often get confused when it comes to more technical details and refractive correction numbers/abbreviations. Here’s a summary and explanation of both.
Myopia occurs when the eyeball is too long from front to back or the cornea (clear front cover of the eye) is steeply curved. As a result, the light entering the eye focuses in the front of the retina (light-sensitive tissue lining the back of the eye), instead of on its surface. In about 10% of people with myopia, the condition progresses in severity. Signs of myopia include:
- Difficulty reading road signs or distant objects clearly
- Poor performance in school
- Eye Strain
- Feeling fatigued when driving or playing sports
- Sitting closer to the television, movie or computer screen
- Rubbing eyes or blinking frequently (children)
Hyperopia occurs when the eyeball is too short from front to back, the cornea is not curved enough, or the lens sits farther back in the eye than normal. As a result, light entering the eye focuses behind the retina, rather than directly on its surface. Farsightedness impacts many infants at birth, however, they grow out of it and far more children end up nearsighted due to environmental factors. Signs of hyperopia include:
- Difficulty performing close tasks like sewing or reading
- Blurred vision, especially at night
- Aching eyes
- Rubbing eyes or blinking frequently (children)
Measuring Visual Acuity and Refractive Errors
Visual acuity is typically performed at the beginning of an eye exam using a standardized Snellen chart placed or projected 20 feet away on a wall. For near vision, a small chart is held 14 inches from the face. Progressively small letters are read, one eye at a time, until you can no longer clearly distinguish them. “Normal” or 20/20 vision means you can clearly read specific letters from 20 feet away. If you have 20/40 vision, this means you can read letters from 20 feet that most people see clearly when they are 40 feet away. The EyeQue Insight is a new at home visual acuity screener that allows anyone ages 6 and above to check clarity of vision in under 3 minutes. This number only indicates visual acuity, so a different measuring device is required to obtain a prescription correction.
Refractive Error Test
A device called a phoroptor or refractor is a quick means of determining refractive error and obtaining an accurate eyeglass prescription. The device contains lenses of different strengths which are moved into the patient’s view. The eye doctor asks which of the two combinations of lenses looks clearer until the best correction is found for each eye. The EyeQue Personal Vision Tracker lets you obtain the same spherical, cylindrical, and axis figures your eye doctor uses to generate a prescription. The Personal Vision Tracker measures the focal length correction needed for light to focus correctly on your retina.
What do All These Numbers Mean?
You have a piece of paper with mysterious numbers and abbreviations, indicating your eyeglass or contact lens prescription, however, you have no idea what this means. A diopter is a standard unit of measurement on eyeglass prescriptions. A negative diopter number specifies nearsightedness and a lens that minimizes things. Conversely, a positive number indicates farsightedness and a lens that magnifies objects. Simply put, a diopter designates how powerful a lens is needed to properly focus light on a person’s retina. A 1.50 to -7.00 diopter signifies mild to moderate myopia. Many people have astigmatism in addition to myopia or hyperopia. In astigmatism, the cornea has a curvature imperfection, or the lens is distorted, resulting in both near and far objects appearing blurry or distorted.
D.V. stands for distance vision and N.V. stands for near vision. O.D. and O.S. are the Latin abbreviations for right and left eye, respectively. Spherical is the measurement in diopters to correct for myopia or hyperopia. Cylindrical is the measurement in diopters to correct for astigmatism. Axis indicates where astigmatism is located along the horizontal axis of the eye. ADD or N.V. is used if you are getting bifocal lenses.
For example, in the spherical column, a -3.25 O.D. corrects for nearsightedness in the right eye, and a +.5 O.S. corrects for farsightedness in the left. The cylindrical and axis numbers for astigmatism are harder to understand. A cylindrical measurement of -1.0 is the additional myopia correction required for a more curved principal meridian. An axis measurement of 90 degrees is the location of the flatter principal meridian, on a 180-degree rotary scale where 90 degrees designates the vertical meridian of the eye, and 180 degrees designates the horizontal meridian.