Contact lens prescriptions (CL Rx) cannot be used directly to purchase eyeglasses because the two prescriptions are not interchangeable. Although both correct vision, the physical placement of the lenses relative to the eye requires different powers, measured in diopters. Using a CL Rx to order glasses risks inaccurate vision correction, which can lead to discomfort and headaches. Understanding these technical differences is necessary to translate your existing data into a safe and accurate spectacle (glasses) prescription (Spectacle Rx).
Why Contact Lens and Glasses Prescriptions Are Different
The fundamental reason for the difference lies in vertex distance, which is the space between the lens and the front surface of the cornea. A contact lens sits directly on the eye’s surface, resulting in a vertex distance of essentially zero. Glasses, however, are positioned about 10 to 14 millimeters away from the eye. This separation significantly alters the effective power of the lens, especially for stronger prescriptions.
The corrective power of a lens is only accurate at the specific distance it was measured. Moving a lens closer or farther changes where the light focuses, effectively changing the lens’s power relative to the eye. For example, a minus-power lens (for nearsightedness) becomes stronger when moved closer, while a plus-power lens (for farsightedness) becomes weaker.
This effect is negligible for prescriptions weaker than \(\pm 4.00\) diopters, where the CL Rx and Spectacle Rx are often the same. For stronger powers, the adjustment, known as vertex distance compensation, is necessary for clear vision. A contact lens power for nearsightedness will always be “less minus” than the corresponding glasses power. The glasses prescription must be calculated to account for the approximately 12mm of space between the spectacle lens and the eye.
Interpreting Your Contact Lens Prescription Data
A contact lens prescription contains multiple data points, some specific to the lens fit and others relevant to glasses. The SPH (Sphere) or PWR (Power) value indicates the main strength needed to correct long or short-sightedness and is the power that requires conversion.
If you have astigmatism, the prescription includes CYL (Cylinder) and AXIS values, which must also be converted. The cylinder indicates the amount of astigmatism correction, and the axis specifies its orientation in degrees. These three measurements—SPH, CYL, and AXIS—describe your refractive error and form the basis for a Spectacle Rx.
Other measurements listed on a CL Rx are unrelated to glasses. The BC (Base Curve), measured in millimeters, describes the curvature on the back of the contact lens to ensure a proper fit on the cornea. Similarly, the DIA (Diameter) indicates the overall width of the contact lens. Since glasses do not sit on the eye, the BC and DIA numbers are irrelevant for obtaining a glasses prescription.
Obtaining an Accurate Spectacle Prescription
The most reliable way to obtain an accurate Spectacle Rx is through a dedicated eye examination for glasses. This exam ensures the prescription is optimized for the specific frame fit and your current visual needs. The eye care professional determines the precise lens power while considering the intended wearing position of the glasses.
If you have a recent contact lens prescription stronger than \(\pm 4.00\) diopters, you can request that your eye doctor perform the vertex distance conversion calculation. This mathematical formula adjusts the lens power from the contact plane (0mm) to the spectacle plane (e.g., 12mm). The prescribing optometrist or ophthalmologist is the only one qualified to perform this conversion accurately, especially when astigmatism is present.
Attempting to self-convert your prescription using online tools is not recommended, as these often lack the precision needed for individual factors. Self-conversion risks inaccurate vision, visual discomfort, and eyestrain. Furthermore, most retail optical providers require a valid Spectacle Rx issued by a licensed eye care professional to fill an order for glasses.