A separate prescription is required for contact lenses, even if you already have one for eyeglasses. This difference exists because eyeglasses and contact lenses function in entirely separate physical environments relative to the eye. A contact lens prescription is a legal authorization for a medical device that must be specifically fitted to your eye’s unique structure. It specifies the optical power and the physical parameters necessary to ensure clear vision and the continued health of the ocular surface.
Why Contact Lens Power Differs From Glasses
The primary reason for a difference in lens power between glasses and contacts is the space between the corrective lens and the eye. Eyeglasses are typically positioned about 12 to 14 millimeters away from the cornea, a distance known as the vertex distance. Contact lenses, by contrast, sit directly on the cornea, resulting in a zero vertex distance. When the lens moves closer to the eye, the optical power, measured in diopters, must be mathematically adjusted to maintain the correct focal point. This conversion is particularly significant for higher prescriptions, generally those exceeding plus or minus 4.00 diopters.
For nearsighted individuals (minus power), the contact lens power will be slightly less than the glasses power. Conversely, for farsighted individuals (plus power), the contact lens power will be slightly greater. This adjustment ensures that the light rays converge at the same point on the retina, delivering the same visual clarity.
Unique Measurements in a Contact Lens Prescription
A contact lens prescription includes specific physical measurements that are irrelevant to a glasses prescription, as they dictate the actual fit of the lens on the eye. The Base Curve (BC) defines the curvature of the back surface of the lens and must closely match the curvature of the cornea. A smaller BC indicates a steeper curve, while a larger number indicates a flatter curve. An incorrect base curve can lead to issues; a lens that is too flat may move excessively and cause irritation, while one that is too steep can restrict tear flow and oxygen supply. The Diameter (DIA) measures the width of the lens from edge to edge, determining how much of the eye’s surface the lens covers.
Patients with astigmatism require Toric contact lenses, which need additional parameters to stabilize the lens’s orientation. These lenses must have a specific Cylinder power and Axis angle to correct the irregular corneal shape. These values ensure the corrective power remains stable and does not rotate, preventing blurred vision.
The Contact Lens Fitting and Evaluation Process
Obtaining a contact lens prescription requires a specialized fitting and evaluation process that goes beyond a standard eye exam. The initial consultation includes a comprehensive eye health assessment and precise measurements of the corneal surface. Specialized equipment is used to map the curvature and dimensions of the cornea, determining the appropriate Base Curve and Diameter. The professional also assesses the quality of the tear film, as dry or unstable tears can significantly impact lens comfort.
Based on these findings, diagnostic or trial lenses are selected and placed on the eye. The professional evaluates the lens’s fit, movement, and centration to ensure proper physiological response. Patients are often sent home with these trial lenses for a period to assess real-world comfort and vision. Mandatory instruction is provided on the proper techniques for lens insertion, removal, and hygienic care before a follow-up appointment is scheduled to finalize the correct lens specifications and issue the legal prescription.
Contact lens prescriptions are generally valid for one year, after which a new evaluation is required to ensure the lenses remain safe and effective.