An eye prescription (Rx) is a standardized document containing measurements that optical professionals use to create corrective lenses. These measurements ensure that light is properly focused onto the retina, the light-sensitive tissue at the back of the eye. Decoding the combination of letters, abbreviations, and numbers on this form can be confusing for most people. The primary confusion lies in determining what the plus or minus signs mean in relation to nearsightedness or farsightedness. This article demystifies these terms to provide a clear understanding of what your prescription indicates.
The Meaning of Positive and Negative Power
The most important number on an eye prescription is found under the “Sphere” (SPH) column, which measures the overall lens power required for correction. This power is measured in Diopters (D) and is always preceded by a plus (+) or a minus (-) sign, indicating the type of refractive error.
A plus sign, such as +2.00 D, indicates farsightedness (hyperopia). People with hyperopia typically see distant objects clearly but struggle to focus on objects up close, which appear blurry. The plus value signifies that the eye requires a lens that adds optical power to bring the focal point forward.
Conversely, a minus sign, such as -2.00 D, signifies nearsightedness (myopia). Individuals with myopia can see near objects clearly, but distant objects appear blurry. The minus value indicates that the eye needs a lens that subtracts optical power to move the focal point back.
The magnitude of the number reflects the severity of the refractive error. For example, a prescription of -5.00 D indicates a greater degree of nearsightedness and requires a more powerful lens than a -1.00 D prescription.
How the Eye Focuses Light
The need for a plus or minus prescription is determined by how the eye’s physical structure causes light to focus incorrectly. In a perfectly sighted eye, the cornea and the lens work together to refract incoming light directly onto the retina. This process creates a sharp image transmitted to the brain.
In nearsightedness (myopia), the eyeball is typically too long from front to back, or the cornea is too steeply curved. Due to this structural difference, light rays focus at a point in front of the retina instead of directly on it. This premature focus causes distant objects to appear blurred.
Farsightedness (hyperopia) usually results from an eyeball that is slightly too short, or a cornea that is too flat. This causes the light rays to focus at an imaginary point behind the retina. While the eye’s natural lens may compensate for mild hyperopia, uncorrected farsightedness generally results in blurry vision for nearby objects.
Astigmatism and Other Common Prescription Terms
Beyond the main spherical power, prescriptions often include terms addressing other vision irregularities. The abbreviations OD and OS are consistently used to distinguish between the right and left eyes, respectively, originating from the Latin terms oculus dexter and oculus sinister. Understanding these labels is necessary for reading the prescription line by line.
Astigmatism occurs when the cornea or lens is shaped irregularly, requiring two specific corrective measurements. The “Cylinder” (CYL) value measures the magnitude of this irregular curvature and the amount of corrective power needed. This power is measured in Diopters and may have a plus or minus sign.
The “Axis” number, ranging from 1 to 180 degrees, specifies the exact orientation or angle of the astigmatism on the eye. The cylinder and axis work together to ensure the lens corrects the vision error along the correct meridian. If astigmatism is absent, the CYL and AXIS columns may be left blank.
Addition (ADD)
“Add” (Addition) is used for people requiring bifocals, trifocals, or progressive lenses, typically to correct presbyopia. Presbyopia is the age-related loss of the eye’s ability to focus on close objects. The ADD number indicates the extra magnifying power needed for reading or near work. This value is almost always positive and is usually the same for both eyes.
Lens Types Used for Correction
The physical shape of the lens is directly determined by the positive or negative sign on the prescription, as the lens must counteract the eye’s focusing error. Lenses for plus (+) prescriptions are designed to converge light rays, meaning they bring parallel light beams closer together. These are known as convex lenses, which are characteristically thicker in the center than at the edges.
The convex lens works by adding the necessary focusing power to move the focal point forward from behind the retina, placing the image directly onto the retina. This correction compensates for the relatively short eye length associated with farsightedness. The light rays are bent inward to achieve the correct focus.
Lenses for minus (-) prescriptions must achieve the opposite effect by spreading light rays apart, a process called divergence. These are concave lenses, which are thinner in the center and thicker at the edges. The concave shape causes light to diverge slightly before entering the eye.
This diverging action effectively pushes the focal point backward, moving the image from its original position in front of the retina to precisely on the retina. The concave lens thus corrects the over-focusing problem resulting from the elongated eyeball of nearsightedness.