Many people who wear glasses and are considering contact lenses assume their existing eyeglass prescription will suffice. Both devices are designed to correct refractive errors like nearsightedness or farsightedness, leading to the belief that the power requirement should be the same. This assumption, however, overlooks the fundamental differences in how each corrective lens sits relative to the eye’s surface. Understanding the distinct requirements for each type of vision correction is necessary for safely wearing contact lenses.
The Requirement for Separate Prescriptions
A separate prescription is required for contact lenses because they are classified as medical devices that rest directly on the eye. This classification makes contact lenses subject to strict regulation to ensure patient health and safety. Federal law mandates that a valid, unexpired contact lens prescription is necessary to purchase lenses, as power correction alone is insufficient for a safe fit.
The physical distance between the corrective lens and the eye is a major factor necessitating two distinct prescriptions. Eyeglass lenses sit approximately 12 millimeters away from the corneal surface, while contact lenses sit directly on the cornea, moving with the eye. This difference in position, known as the vertex distance, changes the effective power needed to focus light onto the retina. For stronger prescriptions, this adjustment often makes the contact lens power slightly weaker than the glasses power.
Specific Measurements for Contact Lenses
A contact lens prescription contains several unique parameters entirely absent from a standard glasses prescription. These geometric measurements are necessary to ensure the lens conforms to the specific shape of the individual eye.
The prescription must include:
- Base Curve (BC): This dictates the curvature of the back surface of the lens and must closely match the curvature of the cornea for a proper fit.
- Diameter (DIA): This specifies the overall width of the lens, ensuring adequate coverage and centering on the cornea.
- Brand/Material: The prescription must specify the brand and material, as different manufacturers use varying materials that affect oxygen permeability and water content.
- Specialized Power Notations: Patients with astigmatism or presbyopia require specific notations for Cylinder (CYL) and Axis or Addition (ADD) power.
The Contact Lens Fitting and Evaluation Process
Obtaining a contact lens prescription involves a specialized process. The process begins with a comprehensive eye exam to determine the refraction and assess overall eye health, including a check for conditions like dry eye or allergies. Specialized instruments are then used to measure the cornea’s curvature, sometimes involving corneal topography for detailed mapping. This data is used to select the initial diagnostic trial lenses.
The patient wears these trial lenses for a short period so the eye care professional can evaluate the fit and movement on the eye. The lens should move slightly with each blink to allow for tear exchange and oxygen flow, but not so much that it causes discomfort or inconsistent vision.
A follow-up appointment is typically required after a trial period to confirm the lens is performing as expected and that the eye health remains stable. Only after this evaluation process is complete and a proper fit is confirmed can a valid contact lens prescription be finalized. The prescription is generally valid for one year, requiring annual checks to ensure the continued safety and comfort of contact lens wear.