An eye prescription is a technical document used to manufacture lenses that correct vision errors. For many people, this document is filled with cryptic abbreviations and numbers. Understanding the core terms, particularly Sphere (SPH) and Cylinder (CYL), is the first step toward demystifying your vision correction needs. These values specify the precise optical power required to bring light into sharp focus on the retina.
The Role of Sphere (SPH)
The SPH column, which stands for “Sphere,” indicates the primary lens power needed to correct general distance vision issues. This value addresses nearsightedness or farsightedness, refractive errors caused by the overall shape of the eyeball. The term “sphere” refers to the fact that this correction is equal across all meridians of the lens.
This power is measured in units called diopters (D), which quantify the lens’s ability to bend light. The sign preceding the number is significant: a minus sign (-) indicates correction for nearsightedness. A plus sign (+) signifies correction for farsightedness, a condition where close-up objects are out of focus.
The magnitude of the number, regardless of the sign, reflects the strength of the required correction. If the SPH column is left blank or reads “Plano” (Pl), it means no spherical correction is needed for distance vision.
Defining Cylinder (CYL) and Axis
The CYL value, short for “Cylinder,” is included only when the eye has astigmatism. Astigmatism is a common condition where the cornea or lens is shaped more like the side of a football than a perfect sphere. This irregular shape causes light to focus unevenly on the retina, resulting in blurred vision. The cylindrical power measures the additional lens power necessary to compensate for this uneven curvature.
Like the sphere power, the cylinder value is measured in diopters, indicating the strength of the correction needed. If this column is blank or contains a dash, it indicates that astigmatism is absent or correction is unnecessary. However, the CYL power is incomplete without the accompanying “Axis” value.
The Axis specifies the orientation of the astigmatism on the eye’s surface, dictating where the cylindrical corrective power must be applied. This number is always a degree value ranging from 1 to 180. The Axis value ensures the cylindrical lens is precisely aligned to neutralize the distortion. The Cylinder and Axis values always function as a pair; one cannot be present without the other for effective astigmatism correction.
Completing the Prescription: Other Key Terms
Beyond the primary SPH and CYL values, a prescription often contains other abbreviations necessary for lens fabrication. The “ADD” power, or Addition, is a supplemental magnifying power included for those who need help with near vision tasks, typically due to age-related loss of focus. This value is used for multifocal lenses, such as bifocals or progressive lenses, and is added to the distance prescription to create a clear reading zone.
Another important measurement is the Pupillary Distance (PD), which is the space in millimeters between the centers of the two pupils. The PD ensures that the optical center of each lens is aligned directly in front of the patient’s pupils. An inaccurate PD can lead to discomfort, eye strain, and reduced effectiveness of the vision correction.