When you receive an eye prescription, the string of numbers and symbols can appear confusing, especially the presence of a minus sign. This negative number is not a measure of poor health but rather a measurement of the lens power required to correct your vision. Understanding this negative value is the first step in managing your refractive error. The number indicates the strength of the corrective lens needed to focus light onto the back of your eye.
Defining the Negative Number: Nearsightedness
A negative number on an eye prescription, such as -2.50, indicates nearsightedness, also known as myopia. This condition is characterized by clear near vision, while objects at a distance appear blurry. The negative sign signifies that the eye requires a diverging lens to push the focus point backward.
Nearsightedness is a common refractive error. Symptoms often include squinting to see distant signs or experiencing eye strain and headaches. Conversely, a positive number on a prescription indicates farsightedness, where light focuses behind the retina.
The Physical Cause of Negative Vision
The cause of negative vision lies in the physical dimensions and shape of the eye. For clear vision, light must focus directly onto the retina, the light-sensitive tissue at the back of the eye. In a nearsighted eye, the incoming light focuses at a point in front of the retina.
This premature focusing is typically due to one of two anatomical reasons. The most common cause is the eyeball being too long from front to back, known as axial length. The second cause involves the cornea, the clear front surface of the eye, having too steep of a curvature. In either scenario, the eye’s refractive power is too strong for its length, causing the image to fall short. This optical error requires a negative-powered lens to spread the light rays before they enter the eye.
Understanding Diopters and Severity
The unit of measurement used for the power of corrective lenses is the diopter, abbreviated as D. This unit quantifies how much a lens bends light, and the number on your prescription represents the power needed to neutralize the focusing error. The farther the negative number is from zero, the stronger the lens correction needed and the more severe the myopia.
Eye care professionals classify myopia into severity ranges based on the diopter value. Mild myopia is typically defined by prescriptions up to -3.00 D. Moderate myopia falls within the range of -3.00 D to -6.00 D, requiring consistent lens wear for most activities. High myopia is classified as -6.00 D or greater and is associated with an increased risk for other eye health issues, such as retinal detachment.
Methods for Correcting Negative Vision
Correction of negative vision involves using lenses that have a negative, or concave, curvature. These lenses are thinner in the center and thicker at the edges, allowing them to diverge the light rays entering the eye. This divergence is calculated to compensate for the eye’s excessive focusing power, ensuring the light converges on the retina.
Corrective lenses are delivered through eyeglasses or soft and rigid contact lenses. For a more permanent solution, refractive surgeries alter the eye’s structure. Procedures like LASIK and PRK use a laser to reshape the cornea’s curvature. By flattening the cornea, these procedures reduce the eye’s overall focusing power, moving the point of focus back onto the retina. Other options, such as Implantable Collamer Lenses (ICL), involve surgically placing an artificial lens inside the eye to correct the refractive error.