A prescription for glasses and one for contact lenses are not interchangeable. While both are designed to correct the same visual problem, such as nearsightedness or farsightedness, the final prescribed power and physical specifications are distinct. A glasses prescription is for a lens positioned a short distance away from the eye, but a contact lens prescription is for a medical device that sits directly on the eye’s surface. This difference in proximity requires changes to the lens power and the inclusion of specific physical measurements for the contact lens itself. Both prescriptions are legal documents that require a complete eye examination and subsequent fitting process by an eye care professional.
Understanding Vertex Distance
The primary optical reason for the difference in lens power between glasses and contacts is a concept called vertex distance. Vertex distance is the physical space between the back surface of the spectacle lens and the front of the cornea, which is typically between 12 and 14 millimeters. When an eye exam is performed, the power is measured at this specific distance.
A contact lens rests directly on the eye’s surface, meaning its vertex distance is essentially zero. Moving a corrective lens closer to the eye changes its effective power because the light rays are focused differently onto the retina. The power listed on a glasses prescription must therefore be mathematically adjusted, or “vertexed,” to determine the correct power for a contact lens.
This adjustment becomes especially significant for higher prescriptions, generally those that exceed plus or minus 4.00 diopters (D). For a person with high nearsightedness (minus power), the contact lens power will be less minus than the glasses power. Conversely, a highly farsighted person (plus power) will require a contact lens power that is more plus than their glasses prescription.
Physical Contact Lens Specifications
A contact lens prescription contains several physical specifications that are entirely absent from a glasses prescription. These unique parameters are required to ensure the lens fits the eye comfortably, safely, and securely. They relate to the physical shape of the lens and its interaction with the sensitive corneal surface.
The Base Curve (BC) is a measurement, typically in millimeters, indicating the curvature of the lens’s back surface that rests on the cornea. This measurement is crucial because it ensures the lens aligns with the natural curvature of the eye. A smaller number, such as 8.2 mm, indicates a steeper curve, while a larger number, like 9.0 mm, indicates a flatter curve.
The Diameter (DIA), also measured in millimeters, specifies the total width of the lens from edge to edge. For soft contact lenses, the diameter usually falls between 13.0 and 14.5 mm. Additionally, the prescription must specify the exact lens brand and material, as these factors determine the oxygen permeability and water content, which are crucial for long-term corneal health. Without these physical measurements, a lens cannot be safely or effectively fitted to the eye.
The Required Contact Lens Fitting
A simple eye examination for glasses is insufficient for obtaining a contact lens prescription because a specialized fitting process is required. A contact lens fitting includes a detailed assessment of the eye’s physiology and health. This process involves measuring the curvature of the cornea using an instrument called a keratometer or a corneal topographer to determine the appropriate Base Curve and Diameter.
The eye care professional also conducts a tear film evaluation to determine if the eyes produce enough quality tears to keep the lenses hydrated and comfortable throughout the day. Trial lenses are then placed on the eye to observe their movement and fit, ensuring the lens is not too tight, which could restrict oxygen, or too loose, which could cause irritation. The lens must move slightly with each blink to allow for tear exchange and oxygen delivery to the cornea.
The prescription is not finalized until the eye care professional confirms the optimal fit, vision, and comfort after a trial period. This comprehensive fitting process is a medical necessity, emphasizing that contact lenses are regulated medical devices that require ongoing professional supervision. The distinct requirements for power conversion and physical fit necessitate a separate contact lens prescription to protect the health and integrity of the eye.