A separate process, often called a contact lens fitting, is required in addition to a standard eye examination. A comprehensive vision appointment assesses the overall health of the eye and determines the necessary lens power. While the basic health check is shared, the contact lens fitting involves distinct procedures and measurements. This specialized evaluation is necessary because contact lenses are classified as medical devices that rest directly on the eye’s surface.
The Foundation of a Standard Eye Exam
A comprehensive eye exam establishes the prescription for glasses by focusing on the optical properties of the eye and its internal health. The most fundamental step is refraction, which determines the precise lens power needed for clear vision at various distances. This is achieved by asking the patient to compare different lens combinations while viewing an eye chart.
The appointment also includes a thorough eye health screening, checking for signs of diseases such as glaucoma, cataracts, and macular degeneration. The resulting glasses prescription is derived solely from the refractive error. It represents the lens power needed to focus light correctly on the retina from a distance of about 12 to 14 millimeters away from the eye, but it does not account for a lens resting directly on the eye’s surface.
Unique Measurements for Contact Lens Fitting
The contact lens fitting requires specialized measurements because the lens physically touches the cornea, making a proper fit essential for eye health. One important procedure is corneal topography, which uses light to map the precise curvature of the eye’s front surface. This detailed mapping ensures the lens has the correct base curve and diameter to fit the eye’s unique shape without causing irritation or interfering with oxygen flow.
Another distinct step is the tear film evaluation, assessing the quantity and quality of the patient’s natural moisture layer. Adequate tear production is necessary for comfortable and safe contact lens wear. Finally, a diagnostic lens fitting involves placing a trial lens on the eye to observe its movement, centering, and performance during a blink. The doctor must confirm the lens allows for tear exchange and maintains clear visual acuity before finalizing the prescription.
Understanding Prescription Nuances and Expiration
The physical placement of the lens creates a necessary difference between the two prescriptions. Eyeglasses are worn at a distance from the cornea, creating a vertex distance, usually around 12 to 14 millimeters. Because a contact lens sits directly on the eye, the power from the glasses prescription must be adjusted to compensate for this change in distance. This power adjustment, often significant for prescriptions stronger than plus or minus 4.00 diopters, ensures the patient receives the same effective correction.
Contact lens prescriptions are legally distinct documents requiring additional specifications compared to those for glasses, such as the specific lens brand, material, base curve, and diameter, due to the risk of eye infection and injury associated with improper use. Contact lens prescriptions also have a different expiration period, with federal law mandating a minimum validity of one year unless a medical reason warrants a shorter duration.