Yes, contact lens prescriptions are different from glasses prescriptions. This difference arises primarily because contact lenses sit directly on the surface of the eye, while eyeglasses are positioned approximately 12 to 14 millimeters away from the eyes. This distinct positioning impacts light refraction and the specific measurements required for each type of vision correction. Therefore, a prescription for one cannot be simply converted or used for the other.
Key Differences in Prescription Measurements
Eyeglass Prescriptions
For eyeglasses, standard measurements include sphere (SPH), which indicates the lens power needed for nearsightedness or farsightedness, measured in diopters. A minus sign denotes nearsightedness, while a plus sign indicates farsightedness. If astigmatism is present, the prescription will also include cylinder (CYL) and axis (AXIS) values. Cylinder measures the lens power required to correct astigmatism, typically a minus number in diopters, while the axis specifies the orientation of this correction in degrees, ranging from 1 to 180.
Pupillary distance (PD), the measurement between the centers of your pupils, is also included to ensure proper lens alignment in the frames. The distance eyeglasses sit from the eye, known as vertex distance, also influences the effective power of the lens, particularly for stronger prescriptions.
Contact Lens Prescriptions
Contact lens prescriptions incorporate all the power measurements found in a glasses prescription, but they also include unique specifications due to their direct contact with the eye. One such measurement is the base curve (BC), which describes the curvature of the back surface of the contact lens, measured in millimeters. This measurement ensures the lens properly matches the curvature of your cornea for a comfortable and stable fit; typical values for soft lenses range from 8.3mm to 9.0mm.
Another specific measurement is the diameter (DIA), which is the overall width of the contact lens from edge to edge, also measured in millimeters. This ensures the lens adequately covers the cornea, with common soft contact lens diameters typically ranging from 13.8mm to 14.5mm. Additionally, contact lens prescriptions often specify the exact brand of the lens, as different manufacturers use varying materials like hydrogel or silicone hydrogel, which affect breathability and fit.
Importance of Separate Eye Exams and Fittings
Contact lenses are not a one-size-fits-all solution, as each person’s eye has a unique shape and curvature. An optometrist or ophthalmologist plays a central role in evaluating overall eye health before prescribing contact lenses, checking for conditions such as dry eye, allergies, or corneal irregularities that might affect lens wear. This comprehensive eye exam also includes assessing tear film quality, which influences the type of contact lens material that would be most comfortable.
The contact lens fitting process involves several precise steps to ensure proper fit, vision, and comfort. During the fitting, trial lenses are often used to assess how the lens sits on the eye, its movement, and how it corrects vision. The eye care professional measures the curvature and diameter of the cornea using specialized instruments like a keratometer or corneal topographer. They also provide instructions on proper insertion, removal, and care, including hygiene practices to minimize risks.
Using an incorrect or unfitted contact lens prescription can lead to various complications. A poorly fitting lens can cause discomfort, irritation, redness, and blurry or inconsistent vision. Potential risks include corneal abrasions (scratches on the eye’s surface), eye infections, and corneal damage. A lens that is too tight can restrict oxygen flow to the cornea, potentially leading to tight lens syndrome, while a loose lens may move excessively, causing irritation or trapping debris.