You cannot use a contact lens prescription directly to order eyeglasses, because the two types of vision correction sit at different distances from the eye. The power listed on a contact lens prescription is calculated for a lens resting directly on the cornea, which is a position known as zero vertex distance. Conversely, the power for glasses is determined for a lens positioned approximately 12 to 14 millimeters away from the eye. This small difference in distance necessitates a mathematical adjustment to the lens power to ensure that light focuses correctly on the retina, preventing blurry vision and eye strain.
Distinct Parameters on Each Prescription
A contact lens prescription contains parameters specific to the physical fit and material of the lens that are irrelevant for glasses. These unique measurements include the Base Curve (BC) and Diameter (DIA). The base curve specifies the curvature of the back surface of the contact lens, ensuring it matches the shape of the cornea for a proper, comfortable fit, typically ranging from 8.2 to 9.0 millimeters. The diameter specifies the overall size of the lens.
Contact lens prescriptions also specify the exact lens brand and material, since oxygen permeability and water content vary significantly between products. None of these fitting parameters are required for spectacle lenses. In contrast, both prescriptions will list the refractive power components: Sphere (SPH) for nearsightedness or farsightedness, Cylinder (CYL) for astigmatism, and Axis (AXIS) for the orientation of the astigmatism correction.
The Role of Vertex Distance
The fundamental difference in power between the two prescriptions is due to a concept called vertex distance, which is the space between the back of the lens and the front surface of the cornea. Contact lenses have a vertex distance of zero because they sit directly on the eye. Eyeglasses typically sit 12 to 14 millimeters away from the eye, which significantly alters the effective power of the lens.
This distance effect is most pronounced for stronger prescriptions, typically those exceeding \(\pm 4.00\) diopters (D). For people who are nearsighted (myopic), the glasses must have a slightly weaker power than the contact lenses to achieve the same visual correction. Conversely, for people who are farsighted (hyperopic), the glasses must have a slightly stronger plus power. Moving a lens further from the eye increases the effective power of a plus lens and decreases the effective power of a minus lens.
Measurements Required for Glasses Only
Eyeglasses require specific measurements that are entirely absent from a contact lens prescription because they pertain to the frame and lens placement. The most important of these is the Pupillary Distance (PD), which is the measurement in millimeters between the centers of the two pupils. This measurement is essential because it ensures the optical center of the lens, where the power is most accurate, is aligned directly in front of the pupil.
A misalignment by even a few millimeters can cause prismatic effects, leading to eye strain, headaches, or blurry vision, especially in higher prescriptions. For multifocal or progressive lenses, the required measurements become even more complex, often needing a monocular PD (the distance from the pupil center to the bridge of the nose for each eye) and a segment height to determine the position of the near-vision portion. Frame size, shape, and how the frame sits on the face also influence the final power calculation, which cannot be determined from a contact lens fitting.
Converting and Verifying the Final Prescription
The conversion from a contact lens prescription to a glasses prescription involves a precise mathematical recalculation of the sphere and cylinder powers to compensate for the vertex distance. Eye care professionals use a specialized formula to determine the necessary power change, which varies based on the original power and the measured distance between the eye and the spectacle lens. The astigmatism correction (cylinder and axis) may also require adjustment, as a toric contact lens for astigmatism can rotate slightly on the eye, which is not a factor for a fixed spectacle lens.
The converted prescription must always be verified by a licensed optometrist or ophthalmologist. The eye care professional confirms the final visual acuity and comfort through a comprehensive eye examination and a final fitting. Furthermore, a new glasses order requires a recent comprehensive eye exam to ensure the underlying eye health is current, as prescriptions typically have expiration dates.