A contact lens prescription (CL Rx) is not directly interchangeable with a glasses prescription (GP Rx), though they are closely related. Many people seek to derive their GP Rx from their CL Rx, often for convenience when ordering new eyewear. While this conversion is possible, it involves specific optical adjustments due to the differing positions of the lenses relative to the eye. Understanding these differences is necessary to ensure the resulting glasses prescription provides clear and comfortable vision. This article details the steps for this conversion.
The Fundamental Difference Between Contact and Glasses Prescriptions
The main reason contact lens and glasses prescriptions differ is a physical measurement known as “vertex distance.” This is the space between the back surface of a corrective lens and the front surface of the eye’s cornea. Contact lenses sit directly on the cornea, giving them a vertex distance of essentially zero. Eyeglass lenses, however, are typically positioned about 12 to 14 millimeters away from the eye. This small distance significantly impacts the effective power of the lens, particularly for stronger prescriptions. Therefore, a mathematical adjustment is necessary to ensure the glasses lens provides the same visual correction that the contact lens does.
Identifying Key Values on Your Contact Lens Prescription
Before any conversion can occur, the specific numerical values from the contact lens prescription must be located and understood. The core components of the optical power are the Sphere (SPH), Cylinder (CYL), and Axis (AX). The Sphere value corrects for nearsightedness or farsightedness and accounts for the majority of the refractive error.
The Cylinder and Axis values are only present if astigmatism correction is needed. Cylinder indicates the strength of the astigmatism correction, while Axis specifies the orientation on the eye (1 to 180 degrees). These three values—Sphere, Cylinder, and Axis—are the only ones that transfer to a glasses prescription. Values such as Base Curve (BC) and Diameter (DIA), which relate to how the contact lens fits the eye’s curvature, should be ignored for glasses conversion.
Calculating the Glasses Prescription Using Vertex Distance
The calculation for converting a contact lens power to a glasses power is primarily focused on adjusting the Sphere value due to the change in vertex distance. This adjustment is typically only necessary when the spherical power is equal to or greater than \(\pm\)4.00 Diopters (D). For lower prescriptions, the optical difference caused by the 12 to 14 millimeter distance is generally small enough to be clinically insignificant, and the spherical power can be used as written.
The adjustment rule is determined by the lens type and the movement of the lens away from the eye. When converting a contact lens power to a glasses power, the lens is effectively moving away from the eye. For prescriptions correcting nearsightedness (minus powers), the glasses prescription will need to be slightly more minus (stronger) than the contact lens prescription.
Conversely, for prescriptions correcting farsightedness (plus powers), the glasses prescription will need to be slightly more plus (stronger) than the contact lens prescription. For example, a contact lens prescription of -6.00 D may convert to approximately -6.37 D for glasses worn at 12mm. A contact lens power of +6.00 D would convert to approximately +6.50 D for glasses. This change occurs because moving a minus lens away from the eye makes it optically weaker, so the glasses lens must be stronger to compensate. The Cylinder and Axis values usually remain the same, as the vertex distance change primarily affects the spherical component of the power.
When You Must Consult an Eye Care Professional
Self-conversion of a contact lens prescription to a glasses prescription is only reliable for simple, low-power cases. Consulting an eye care professional is mandatory in several specific scenarios.
High Cylinder Values
If the contact lens prescription includes a high cylinder value, generally greater than -2.00 D, a professional fitting is recommended. Astigmatism correction in glasses can sometimes differ from that in contacts.
Complex or High Spherical Powers
Complex prescriptions, such as those with spherical powers above \(\pm\)6.00 D, require precise calculations best handled by a professional using specialized equipment.
Prism Correction
If the glasses prescription needs to include a Prism correction, a new examination is required. Prism correction is never present on a standard contact lens prescription. A dedicated eye examination also ensures the optometrist can verify the final visual acuity and comfort based on the chosen frame and its specific vertex distance.