The desire to use a current contact lens prescription (CL Rx) to purchase eyeglasses is common, offering a convenient starting point for vision correction needs. While the contact lens prescription contains much of the necessary information, it is not directly interchangeable with a glasses prescription (GP Rx). A glasses prescription is a translation of the contact lens data, requiring specific adjustments to power values to ensure clear vision. Understanding the physical and mathematical reasons for this difference is necessary to successfully convert the prescription.
Decoding Your Prescription Numbers
Both contact lens and glasses prescriptions utilize standardized numerical values to describe the required vision correction. The core power measurement is the Sphere (SPH or PWR), measured in Diopters (D), which corrects for nearsightedness (minus sign) or farsightedness (plus sign). Astigmatism correction is detailed by two additional values: Cylinder (CYL) and Axis (AXIS). The Cylinder measures the severity of the astigmatism in Diopters, and the Axis indicates the orientation on the eye (1 to 180 degrees).
The contact lens prescription includes unique parameters irrelevant for glasses, such as the Base Curve (BC) and Diameter (DIA). The Base Curve defines the curvature of the back surface of the contact lens to match the eye’s shape. These physical measurements are specific to the lens material and brand and do not transfer to a glasses prescription, which relies only on power measurements.
Why the Numbers Change: Understanding Vertex Distance
The fundamental reason a contact lens prescription differs from a glasses prescription is the concept of vertex distance. This is the space between the back surface of a corrective lens and the front of the eye’s cornea, typically 12 to 14 millimeters (mm) for standard eyeglasses. A contact lens sits directly on the cornea, making its vertex distance zero.
This difference in positioning significantly impacts the effective optical power required to focus light onto the retina. This distance effect is negligible for low-strength prescriptions, specifically those below plus or minus 4.00 Diopters (±4.00 D). For powers stronger than this threshold, the change in distance necessitates a power adjustment.
Step-by-Step Conversion of Sphere Power
Converting spherical power involves adjusting for the vertex distance, which is necessary only when the power is \(\pm\)4.00 D or greater. When moving from a contact lens (zero distance) to a glasses lens (12-14 mm distance), the required power for nearsighted (minus) prescriptions must be reduced. This means a \(-6.00\) D contact lens power will translate to a slightly weaker minus power for glasses, such as \(-5.75\) D or \(-5.50\) D.
For farsighted (plus) prescriptions, the power must be increased when moving from a contact lens to a glasses prescription. A contact lens power of \(+6.00\) D, for example, requires a stronger power like \(+6.25\) D or \(+6.50\) D in the glasses lens. This adjustment is based on a specific optical formula (Fc = Fg / (1 – dFg), where Fc is the contact lens power, Fg is the glasses power, and d is the vertex distance in meters).
Most people rely on conversion charts or online calculators based on a standard 12 mm vertex distance rather than manual calculation. For instance, a \(-8.00\) D contact lens power converts to approximately \(-7.25\) D for glasses, and a \(-10.00\) D contact lens converts to about \(-8.75\) D. The final power must always be verified by an eye care professional, particularly for high powers.
Addressing Astigmatism and Near Vision (ADD Power)
The correction for astigmatism, defined by the Cylinder (CYL) power, also requires a vertex distance adjustment, similar to the spherical power. If the spherical power warrants conversion, the cylinder power must also be converted using the same vertex distance formula. The Axis (AXIS) value, which indicates the orientation, is a geometric measurement and remains the same when converting to glasses.
Contact lenses often come in a limited range of Cylinder powers, usually rounded to steps like \(-0.75\) D, \(-1.25\) D, or \(-1.75\) D. The corresponding glasses prescription may require a slightly different, more precise Cylinder power to achieve full correction. This difference means the glasses prescription often provides a more exact correction for astigmatism than the contact lens equivalent.
For individuals requiring multifocal or bifocal correction, the Near Vision Addition (ADD power) is transferred directly without adjustment. The ADD power relates to the eye’s reduced ability to focus up close due to age, a condition called presbyopia. Since the ADD power accounts for the eye’s internal focusing mechanism, its required strength is independent of the lens’s distance from the eye.