A glasses prescription provides the necessary optical power to correct vision. However, that prescription cannot be used directly to purchase contact lenses, even though the underlying vision correction is the same. This is because contact lenses require specific measurements and professional assessments, accounting for the lens sitting directly on the eye. The transition from a spectacle lens, which rests about 12 to 14 millimeters away, to a contact lens, which sits on the cornea, changes the effective power needed for clear sight. This difference requires a calculated adjustment to the lens strength and physical measurements to ensure a comfortable and safe fit.
The Impact of Vertex Distance
The primary mathematical reason a glasses prescription cannot be used for contacts involves a concept called vertex distance. This is the space measured between the back surface of the spectacle lens and the cornea, typically ranging from 12 to 14 millimeters when glasses are worn. Since a contact lens sits directly on the eye, its vertex distance is effectively zero. This shift in distance alters the effective power required to focus light precisely on the retina, necessitating an adjustment to the prescription power.
This power adjustment is most significant for higher prescriptions, which are generally considered to be at or beyond plus or minus 4.00 diopters. When converting a strong minus (nearsighted) glasses prescription, the contact lens power must be calculated to be weaker. Conversely, a strong plus (farsighted) prescription requires a stronger contact lens power. Ignoring this calculation, known as vertex conversion, can lead to blurry vision, headaches, or eye strain.
Essential Measurements Unique to Contact Lenses
Beyond the power calculation, contact lenses require physical measurements that are completely absent from a standard glasses prescription. These parameters ensure the lens fits the eye’s surface correctly, promoting comfort and maintaining eye health. The two most important metrics are the Base Curve (BC) and the Diameter (DIA).
The Base Curve refers to the curvature of the back surface of the contact lens, measured in millimeters. It must closely match the curvature of the user’s cornea. A typical soft contact lens BC usually falls between 8.3 and 9.0 millimeters. If the base curve is too flat or too steep, the lens will move too much or sit too tightly. This causes discomfort, irritation, and potentially restricts the eye’s necessary oxygen supply.
The Diameter is the overall width of the lens, measured edge-to-edge, and is typically between 13.0 and 14.5 millimeters for soft lenses. The correct diameter ensures the lens covers the cornea appropriately without interfering with the eyelid margin or causing excessive movement. The proper combination of the Base Curve and Diameter determines the lens’s overall fit and stability on the eye.
Why a Professional Contact Lens Exam is Required
Obtaining a contact lens prescription necessitates a specialized professional exam and fitting process, which is legally separate from a glasses prescription. This specialized appointment includes measuring the corneal curvature with instruments like a keratometer to determine the ideal Base Curve and Diameter. The eye care professional also assesses the eye’s overall health, looking for conditions that could be aggravated by lens wear, such as dry eye syndrome.
During the fitting, trial lenses are placed on the eye so the doctor can observe the lens movement, centering, and how it interacts with the tear film. This real-time evaluation ensures the lens allows for adequate oxygen flow to the cornea, which is vital for long-term eye health. The final, legally binding contact lens prescription will include the specific lens power, Base Curve, Diameter, brand name, material type, and a recommended wear schedule.