An eye prescription is essentially a precise set of instructions used by an optician to create lenses that will correct your vision. This document translates the specific measurements of your eyes into the lens powers needed for clarity. The three primary components defining this necessary corrective power are the Sphere (SPH), the Cylinder (CYL), and the Axis. Each element addresses a different type of refractive error, ensuring that light focuses properly onto the retina.
Sphere: The Power for Overall Vision Correction
Sphere Definition
The Sphere (SPH) is the foundational measurement on a prescription, indicating the main lens power required to correct basic focusing errors. This value addresses conditions where the eye is either too long or too short, causing light to focus incorrectly. A spherical lens applies a uniform amount of power across its entire surface, much like a slice taken from a perfectly round ball.
Correcting Myopia and Hyperopia
The SPH value corrects for both myopia, or nearsightedness, and hyperopia, or farsightedness. Myopia occurs when the light focuses in front of the retina, and this is corrected by a negative SPH value. Conversely, hyperopia happens when the light focuses behind the retina, requiring a positive SPH value to shift the focal point forward.
Cylinder and Axis: The Astigmatism Correction
Cylinder (CYL)
The Cylinder (CYL) and Axis measurements are always presented together on a prescription because they work in tandem to correct astigmatism. Astigmatism is a common condition where the cornea or the lens inside the eye is shaped more like a football rather than a perfect sphere, causing light to focus unevenly. The CYL value measures the magnitude or strength of the extra lens power needed to neutralize this uneven curvature. If the CYL column is blank, it means you have no measurable astigmatism.
Axis Orientation
The Axis measurement dictates the precise orientation or angle where the cylindrical power must be applied on the lens. Since astigmatism is an irregularity of shape, the corrective power must be oriented to counteract the specific tilt of the eye’s curvature. The Axis is measured in degrees, ranging from 1 to 180. This numerical angle ensures the cylindrical lens is perfectly aligned with the astigmatic curve of the eye.
Interpreting the Prescription Numbers and Signs
Diopters and Strength
The numerical values listed under Sphere and Cylinder are measured in a unit called Diopters, often abbreviated as ‘D’. A Diopter is a unit of optical power that describes how much a lens converges or diverges light. The farther the numerical value is from zero, regardless of the sign, the stronger the prescribed correction is considered to be.
Understanding the Signs
The sign preceding the Diopter value provides immediate information about the type of refractive error being corrected. A minus sign (-) indicates a correction for myopia, or nearsightedness, which requires a diverging lens to push the focal point backward onto the retina. A plus sign (+) signifies a correction for hyperopia, or farsightedness, utilizing a converging lens to pull the focal point forward. The signs in the Cylinder column follow the same convention for the type of astigmatism.