The answer to whether you need a fitting for contact lenses is definitively yes, as these are classified as medical devices requiring professional oversight. Unlike eyeglasses, which sit away from the surface of the eye, contact lenses rest directly on the delicate corneal tissue. Because of this direct contact, the lenses must be perfectly matched not only to your visual needs but also to the unique physical shape of your eye. A proper fitting by an eye care professional ensures the lenses provide clear vision without compromising the long-term health of your eyes.
The Difference Between Glasses and Contact Lens Prescriptions
A prescription written for eyeglasses is insufficient for purchasing contact lenses because it is designed for a lens positioned approximately 12 millimeters away from the eye. This distance, known as the vertex distance, significantly affects the optical power required to correct vision. When a lens is placed directly on the eye’s surface, the required power must be adjusted, often resulting in a slightly different prescription strength than what is listed for glasses.
The glasses prescription primarily provides the refractive error measurements, such as sphere, cylinder, and axis. It completely lacks the parameters needed to ensure the lens physically conforms to the curvature and size of the cornea. For example, a contact lens prescription must include the base curve and diameter, measurements that are irrelevant for spectacles.
Contact lenses are medical devices because they interact directly with the eye’s biology and environment. The prescription must specify the exact brand and material, as different lenses permit varying levels of oxygen flow. This oxygen flow is a factor that must be carefully managed for corneal health.
Key Ocular Measurements Taken During a Fitting
The contact lens fitting process involves taking several precise measurements to ensure the lens matches the eye’s unique topography. The primary measurement is the Base Curve (BC), which defines the curvature of the lens’s back surface in millimeters. This measurement must closely align with the curvature of the cornea to ensure the lens rests gently on the eye without being too tight or too loose.
The eye care professional uses a device called a keratometer or a corneal topographer to map the corneal surface. The keratometer measures the curvature of the central cornea, while topography creates a detailed map of the entire front surface of the eye, useful for irregular shapes like astigmatism. This mapping ensures the selected lens will not cause discomfort or compromise vision stability.
Another crucial measurement is the Diameter (DIA), which specifies the width of the lens from edge to edge. The diameter determines how much of the eye’s surface the lens will cover, affecting both stability and comfort. Following these measurements, a trial lens is placed on the eye to evaluate its movement, centering, and visual clarity.
The fitting also includes an assessment of the tear film, the thin layer of fluid that lubricates the eye. Poor tear quality or insufficient production can affect lens comfort and necessitate a specific lens material designed for dry eyes. The final prescription is issued only after confirming the optimal combination of power, base curve, diameter, and material is achieved.
Health Risks Associated with Improperly Fitted Lenses
Skipping a professional fitting and wearing an improperly sized contact lens can lead to several specific health complications. A lens that is too tight can restrict the flow of tears and oxygen to the cornea, a condition known as hypoxia. This oxygen deprivation can lead to corneal swelling (edema) and, in severe cases, cause new blood vessels to grow into the cornea, a condition called neovascularization.
A tight lens also traps debris and microorganisms beneath its surface, significantly increasing the risk of severe eye infections like keratitis and corneal ulcers. Corneal ulcers are the most severe complication of contact lens wear and can lead to extensive scarring or permanent vision loss if not treated immediately. Symptoms often include pain, light sensitivity, and redness.
Conversely, a lens that is too loose will move excessively with each blink, leading to discomfort and fluctuating vision. This constant friction can cause a corneal abrasion, a painful scratch on the cornea. An untreated corneal abrasion is vulnerable to infection, highlighting the necessity of a lens that moves just enough for tear exchange without causing mechanical trauma.