Both glasses and contact lenses correct vision, but the required eye examinations are not identical. A single appointment often combines two distinct components: a comprehensive eye health check and a specialized contact lens fitting. The shared steps ensure the eyes are healthy and identify the basic refractive error. The unique steps focus on the physical measurements and safety requirements necessary for wearing a medical device directly on the eye.
The Comprehensive Eye Health Check
The comprehensive eye health check is the foundational part of any eye appointment, regardless of the patient’s preferred vision correction method. This common set of procedures establishes a baseline for overall visual health and determines the magnitude of any refractive error. The process begins with visual acuity testing, where a patient reads letters from a chart to measure how clearly they see at a distance.
This initial assessment is followed by a refraction test, a subjective process using a device called a phoropter. The patient compares different lens combinations to find the sharpest focus, which determines the spherical and cylindrical power needed to correct nearsightedness, farsightedness, or astigmatism. Every comprehensive exam includes a detailed health screening to check for conditions affecting the front and back of the eye. This involves measuring intraocular pressure to screen for glaucoma and using a slit lamp microscope to examine structures like the cornea, lens, and retina.
Unique Procedures for Contact Lens Fitting
The contact lens fitting is a specialized set of procedures performed in addition to the comprehensive eye check, as lenses are medical devices that sit directly on the corneal surface. A primary requirement is measuring the precise curvature of the cornea, the clear front dome of the eye. This is done using a keratometer or corneal topography, which creates a detailed map of the eye’s shape.
These measurements determine the correct base curve and diameter of the contact lens, essential for a safe and comfortable fit. If a lens is too flat or too steep, it can cause discomfort, blurry vision, or damage the eye by restricting oxygen flow or rubbing. The eye doctor must also evaluate the quality and quantity of the patient’s tear film, often using specialized dyes, because inadequate tear production can lead to dryness and intolerance when wearing lenses. Finally, a trial lens is placed on the eye to observe its physical movement and position, confirming that the lens centers correctly and moves slightly with each blink for proper tear exchange.
Differences in Prescription Values and Renewal
The final prescription written for a contact lens differs from a glasses prescription due to vertex distance. Vertex distance is the space between the lens and the cornea, typically 12 to 14 millimeters for glasses but zero for contact lenses. This difference changes the effective power of the lens, requiring a mathematical adjustment called vertex compensation, particularly for prescriptions stronger than plus or minus 4.00 diopters.
The contact lens prescription must also include specific physical attributes that a glasses prescription does not, such as the base curve, diameter, lens material, and specific brand name. Contact lenses are regulated as medical devices, which legally mandates a renewal process, typically every year. This annual renewal ensures the physical fit of the lenses and the overall health of the eyes are regularly monitored, protecting the patient from potential complications.