When switching from contact lenses to glasses, the prescription values must be converted. A contact lens prescription (CL Rx) must be adjusted to a glasses prescription (GP Rx) because the lens location relative to the eye changes the perceived optical power. This conversion ensures the glasses provide the same focused vision as the contacts. The adjustment is necessary due to the physical separation between the eyeglass lens and the cornea.
Deciphering Your Prescription Terms
Understanding the terminology on your contact lens prescription is the first step. Corrective power is measured in diopters (D) and listed under Sphere (S or SPH). A minus sign (-) indicates nearsightedness (myopia), and a plus sign (+) indicates farsightedness (hyperopia).
For astigmatism, two additional values are present: Cylinder (Cyl), which specifies the corrective power for the irregular corneal shape, and Axis, which indicates the orientation of that power (from 1 to 180 degrees). OD (Oculus Dexter) and OS (Oculus Sinister) specify the right and left eye. Measurements like Base Curve and Diameter relate only to the physical fit of the contact lens and are irrelevant for glasses.
The Role of Vertex Distance in Power Change
Prescription conversion is necessary due to vertex distance. This is the physical space between the back surface of the corrective lens and the front surface of the cornea. A contact lens rests directly on the cornea, resulting in a vertex distance of approximately zero millimeters.
Glasses are typically positioned 12 to 14 millimeters away from the cornea. Changing this distance alters the point where light focuses on the retina, thereby changing the effective power perceived by the eye. For example, moving a minus lens (for myopia) closer increases its effective power, while moving a plus lens (for hyperopia) closer decreases its effective power. This difference in placement requires a mathematical adjustment to the Sphere power. The effect of vertex distance becomes significant for powers greater than \(\pm 4.00\) diopters.
Step-by-Step Spherical Conversion Calculation
The calculation to convert contact lens power to glasses power is called vertex distance compensation. This adjustment is required for the spherical component of any prescription exceeding \(\pm 4.00\) D. The formula calculates the new glasses power (\(F_g\)) from the contact lens power (\(F_c\)): \(F_g = F_c / (1 – d \times F_c)\), where \(d\) is the change in vertex distance in meters. A standard distance of \(12\) millimeters (\(0.012\) meters) is typically used for this conversion.
For example, consider a myopic contact lens prescription of \(-8.00\) D. Applying the formula results in a glasses power of approximately \(-8.85\) D, which is typically rounded to the nearest quarter diopter, or \(-8.75\) D. Conversely, a hyperopic prescription of \(+6.00\) D converts to about \(+6.46\) D, rounded to \(+6.50\) D. In both examples, the glasses prescription is numerically greater in magnitude than the contact lens power, a consistent pattern when the lens is moved away from the eye.
Dealing with Cylinder, Axis, and High Prescriptions
Astigmatism correction, indicated by Cylinder and Axis values, adds complexity to the conversion. When a prescription includes cylinder, vertex distance compensation must be applied independently to both principal meridians of power, not just the Sphere component. This process, known as meridional vertexing, involves converting the combined Sphere and Cylinder powers at their respective axes and then re-combining the results into the new glasses format.
Prescriptions involving significant cylinder power are often best managed using specialized online calculators or by an eye care professional. Professional guidance is also important for high powers, typically those above \(\pm 6.00\) D. Small errors in manual calculation for strong prescriptions can lead to noticeable visual blur or discomfort.