An eye prescription is a document from an eye care professional, detailing the specific lens requirements to correct vision. It acts as a blueprint for creating corrective lenses, allowing light to focus properly within the eye. Understanding its components, particularly what “power” signifies, helps individuals comprehend their vision correction needs and how their glasses or contact lenses will function.
The Concept of Lens Power
Lens power, a fundamental measurement on an eye prescription, quantifies the strength of the corrective lens needed to focus light onto the retina. It is measured in diopters (D). A higher diopter number, whether positive or negative, indicates a stronger lens is required for a greater refractive error. Corrective lenses work by bending incoming light rays before they enter the eye. This bending ensures light converges directly on the retina, the eye’s light-sensitive tissue, to produce a clear image.
Differentiating Nearsightedness and Farsightedness
The positive (+) or negative (-) signs preceding the diopter number under the “Sphere” (SPH) column on an eye prescription are significant. A negative (-) diopter value indicates nearsightedness (myopia). In nearsightedness, the eye focuses light in front of the retina, causing distant objects to appear blurry while close objects remain clear. Corrective lenses with a negative power spread out the light rays, pushing the focal point back onto the retina.
Conversely, a positive (+) diopter value indicates farsightedness, or hyperopia. With farsightedness, the eye focuses light behind the retina, leading to blurry vision for near objects. Lenses with a positive power converge light rays, pulling the focal point forward onto the retina to improve clarity for close-up tasks. The magnitude of the diopter number, whether positive or negative, reflects the extent of either nearsightedness or farsightedness.
Addressing Astigmatism with Cylinder and Axis
Astigmatism is a common vision condition caused by an irregular curvature of the eye’s cornea or lens, which prevents light from focusing evenly on the retina. This irregularity can make vision appear blurred or distorted at all distances.
To correct astigmatism, eye prescriptions include “Cylinder” (CYL) and “Axis” (AX) values. The “Cylinder” value specifies the amount of additional lens power required to compensate for this uneven curvature. The “Axis” value, measured in degrees from 1 to 180, denotes the precise orientation or angle at which this cylinder power should be applied. The axis is crucial for aligning the corrective power correctly with the eye’s irregular shape, and without it, the astigmatism would remain unaddressed. These cylinder and axis measurements will only appear on a prescription if astigmatism is present and requires correction.
Interpreting Your Complete Eye Prescription
An eye prescription uses standardized abbreviations for each eye. “OD” (oculus dexter) refers to the right eye, while “OS” (oculus sinister) denotes the left eye. The “SPH” column indicates the spherical lens power for nearsightedness or farsightedness. The “CYL” column lists the cylindrical power for astigmatism, and the “AXIS” specifies its orientation. If you do not have astigmatism, these columns may be blank or contain “PL” (Plano), indicating no cylindrical correction is needed.
Beyond these values, some prescriptions may include “ADD” (addition) power, typically for multifocal lenses to assist with near vision, especially for presbyopia. A “Prism” value, measured in prism diopters, might also be present if there are eye alignment issues causing double vision. For example, a prescription might read: OD -2.50 SPH -1.00 CYL 90 AXIS; OS -2.00 SPH. This indicates the right eye needs -2.50 diopters for nearsightedness and -1.00 diopters of cylindrical correction for astigmatism oriented at 90 degrees, while the left eye needs -2.00 diopters for nearsightedness with no astigmatism correction.