The idea that a person can use a contact lens prescription directly to purchase eyeglasses is a common misunderstanding. While both prescriptions contain numbers for visual correction, they are not interchangeable medical documents. The difference stems from the physical placement of the lens relative to the eye, which significantly alters the required optical power. An eye care professional must perform a conversion to ensure the vision correction is accurate for the new type of lens.
Why Contact and Glasses Prescriptions Are Not Interchangeable
The core reason these prescriptions must differ is the fundamental difference in where the corrective lens is positioned. Eyeglasses sit in a frame, placing the lens approximately 12 to 14 millimeters away from the eye’s cornea. Conversely, a contact lens rests directly on the tear film of the cornea, eliminating this distance entirely. This separation, known as the vertex distance, changes how light is bent before it reaches the retina.
The difference in distance affects the required optical power needed for clear vision. Because the two devices are classified as distinct medical devices, a prescription written for one is legally valid only for that specific type of device. Using a contact lens prescription for glasses would result in a vision correction that is optically incorrect, especially for higher powers.
Measurements Unique to Contact Lenses
A contact lens prescription includes several measurements absent from an eyeglasses prescription, as they relate to the physical fit of the device on the eye’s surface. One unique parameter is the Base Curve (BC), which measures the curvature of the back surface of the contact lens. This value must closely match the natural curvature of the cornea to ensure the lens rests properly and comfortably.
Another required measurement is the Diameter (DIA), which specifies the width of the lens from edge to edge. The Diameter ensures the lens covers the appropriate area of the cornea and sclera, preventing irritation and ensuring proper movement on the eye. A contact lens prescription must also specify the exact Manufacturer and Brand of the lens. This is necessary because lens materials vary greatly in breathability and water content, which affects oxygen flow and overall eye health.
The Vertex Distance Adjustment for Eyeglasses
The most significant optical difference between the two prescriptions is the power adjustment required by the vertex distance. Eyeglasses are positioned at an average vertex distance of 12 millimeters from the eye, while contacts have a vertex distance of zero. This distance changes the effective power of the lens, requiring a mathematical conversion of the Sphere (SPH) and Cylinder (CYL) values.
This power adjustment is particularly important for prescriptions stronger than +/- 4.00 Diopters. For a person with nearsightedness (minus power), the contact lens power will be weaker than the required glasses power because the contact sits closer to the eye. Conversely, for a person with farsightedness (plus power), the contact lens power will be stronger than the glasses power.
Eye care professionals use a specific formula or a conversion chart to calculate the exact power needed for the glasses, ensuring the focal point remains the same despite the change in lens position. Because this calculation involves a change in the actual corrective values, a new, separate prescription is almost always generated for the purchase of eyeglasses.