Prescriptions for eyeglasses and contact lenses are not the same, even though both correct the same refractive errors like nearsightedness or farsightedness. While the core measurements—Sphere (SPH), Cylinder (CYL), and Axis—are shared, the final power values and additional parameters differ significantly. This difference stems from how each lens sits relative to the eye’s surface. Eyeglasses are positioned a short distance away, but contact lenses rest directly on the cornea, which changes the optical power needed for clear vision.
The Critical Role of Vertex Distance
The difference in power between the two prescriptions is due to a concept called vertex distance, which is the physical space between the back surface of a corrective lens and the front surface of the cornea. For eyeglasses, this distance is typically 12 to 14 millimeters, the standard used during a vision test with a phoropter machine.
Contact lenses sit directly on the cornea, making their effective vertex distance zero. This difference changes how the light is bent before it enters the eye, requiring an adjustment to the lens’s power to provide the same visual clarity. For people with mild prescriptions, this conversion is often negligible and may not change the written power value.
The need for a calculated conversion becomes significant when the prescription power exceeds plus or minus 4.00 diopters. For a nearsighted person (minus power), the contact lens power must be weaker than the eyeglass power. Conversely, for a farsighted person (plus power), the contact lens power must be stronger.
Measurements Unique to Contact Lenses
A contact lens prescription requires specific measurements that ensure the lens fits safely and comfortably on the eye. The two unique parameters are the Base Curve (BC) and the Diameter (DIA). The Base Curve dictates the curvature of the back surface of the contact lens, which must closely match the curvature of the individual’s cornea.
The Base Curve is measured in millimeters; a lower number indicates a steeper, more curved lens, while a higher number indicates a flatter lens. If the Base Curve is too flat or too steep, the lens will not fit properly, which can cause discomfort, blurred vision, and potentially damage the eye’s surface.
The Diameter specifies the overall width of the lens, ensuring it covers the appropriate area of the cornea for stable placement. Contact lens prescriptions also specify the exact brand and material. Different materials have varying degrees of oxygen permeability and water content, which affects the health of the eye and the wearing schedule.
Components Specific to Eyeglass Lenses and Frames
Eyeglass prescriptions include their own set of measurements related to the frame and the wearer’s face. The most important of these is the Pupillary Distance (PD), which is the measurement in millimeters between the centers of the two pupils. This number ensures the optical center of each lens is ground exactly where the eye looks through it.
An inaccurate Pupillary Distance can cause eye strain, fatigue, and blurry vision, particularly with stronger prescriptions. Beyond the optical measurements, an eyeglass prescription often includes details about the physical frame, such as frame size and the need for specific lens enhancements. These enhancements can include anti-reflective coatings, photochromic tints, or specialized lens material indices that affect the lens’s thickness and weight.
Legal Necessity of Separate Prescriptions
Legally, one prescription cannot be substituted for the other, even if the power values were identical. Contact lenses are classified by regulatory bodies, such as the U.S. Food and Drug Administration (FDA), as medical devices because they sit directly on the eye. This classification imposes strict safety and regulatory requirements.
Due to the device classification and the risk of eye complications from a poor fit, contact lens prescriptions must be current and include the specific fitting parameters. Federal regulations require prescribers to release a copy of the contact lens prescription to the patient after a successful fitting. Sellers are prohibited from dispensing lenses without a valid, non-expired prescription. This legal framework ensures that patients receive an annual eye health check and fitting evaluation.