When you receive a vision correction prescription, it is important to know whether it is for contact lenses or eyeglasses because the two are almost never the same. A contact lens prescription is not automatically interchangeable with an eyeglass prescription due to how each lens sits relative to the eye.
The Fundamental Difference: Why Prescriptions Change
The physical separation between the corrective lens and the eye’s surface is the primary reason prescriptions diverge. This distance is known as the “vertex distance,” and it significantly impacts how the lens focuses light onto the retina. Eyeglasses are typically positioned about 12 to 14 millimeters in front of the cornea, which is the clear outer layer of the eye.
Contact lenses, however, sit directly on the corneal surface, effectively creating a vertex distance of zero. Because the distance between the corrective lens and the eye changes, the required power of the lens must also change to achieve the same visual result. The power written on the prescription is the measurement of how strongly the lens bends light, and this bending effect is altered by the change in physical position.
Deciphering the Contact Lens Prescription
A contact lens prescription contains several parameters that are specific to the physical fit of the lens on the eye, which are irrelevant for eyeglass wear. The Base Curve (BC) defines the curvature of the back surface of the contact lens, ensuring it matches the shape of the cornea for comfort and proper movement. Similarly, the Diameter (DIA) specifies the width of the lens necessary to cover the cornea correctly.
These fitting measurements, along with the specific lens Material and Brand, have no bearing on the optical power needed for glasses. Only the optical measurements—the Power (PWR or SPH), the Cylinder (CYL), and the Axis (AX)—are relevant for determining the corresponding eyeglass prescription. The Power value, often listed as SPH, represents the primary focus correction and is the main number requiring adjustment for glasses.
Converting Spherical Power: The Vertex Distance Adjustment
The most direct mathematical adjustment between the two prescriptions involves the spherical power, which is the measurement in Diopters (D) used to correct nearsightedness or farsightedness. This conversion is only necessary when the spherical power is approximately plus or minus 4.00 Diopters or stronger. For lower prescriptions, the change in effective power from the vertex distance is generally considered negligible.
The general rule for conversion depends on the sign of the power. If the contact lens prescription is a high minus power, correcting nearsightedness (myopia), the corresponding eyeglass prescription will be numerically weaker, meaning the number will be closer to zero. This is because moving the lens further from the eye increases its effective minus power, so a weaker power is needed in the glasses to compensate.
Conversely, if the contact lens prescription is a high plus power, correcting farsightedness (hyperopia), the corresponding eyeglass prescription will be numerically stronger, meaning the number will be further from zero. Moving a plus lens away from the eye makes its effective power weaker, so the glasses must start with a stronger power to achieve the same focus. For example, a contact lens power of -8.00 Diopters, when converted for a typical 12-millimeter vertex distance, would result in an eyeglass prescription closer to -7.37 Diopters. This difference in effective power can cause eye strain if the wrong correction is worn. Specialized tables or a specific mathematical formula known as the vertex distance formula are used to calculate this precise adjustment.
Handling Astigmatism, Axis, and Multifocal Lenses
Converting the spherical power is only one part of creating an accurate eyeglass prescription, especially when astigmatism is present. Astigmatism is corrected by the Cylinder (CYL) power and the Axis (AX), which indicate the magnitude and orientation of the non-spherical curvature of the eye. Contact lenses designed to correct astigmatism, often called toric lenses, frequently use standardized or rounded cylinder powers that may not exactly match the eye’s precise need.
For instance, a contact lens might only be available in -0.75 D or -1.25 D cylinder powers, even if the eye requires a -1.00 D correction. Eyeglasses, however, can be manufactured with much finer precision, often leading to a different, usually higher, cylinder value on the finalized eyeglass prescription. The Axis measurement, which ranges from 1 to 180 degrees, is also much more sensitive for glasses than for contacts.
Eyeglass lenses remain fixed in place, requiring the axis to be aligned perfectly to correct the astigmatism. Toric contact lenses, conversely, are designed to stabilize on the eye, but they may still rotate slightly, which is a factor the optometrist accounts for when selecting the lens.
For individuals requiring correction for reading, known as presbyopia, both contact lenses and glasses may include an Add Power for near vision. This Add Power generally remains the same between the two prescriptions. However, the overall design of multifocal or bifocal glasses, which involves blended power zones, differs significantly from the structure of multifocal contact lenses, which often rely on concentric rings or specific optical zones.
Because of these many complexities, particularly involving cylinder and axis, self-converting a contact lens prescription should only serve as an estimate. A final, comprehensive eyeglass prescription must be confirmed by an eye care professional.