Many people feel that their prescription glasses provide a stronger or different visual correction than their contact lenses. This perceived difference is not due to an incorrect prescription, but rather a direct consequence of optical physics. The way light is refracted depends heavily on the lens’s position relative to the eye. The physical gap between glasses and contacts causes the optical power to change, which is why the visual experience differs. Understanding this involves looking at the specific geometry of lens placement.
The Critical Factor of Vertex Distance
The primary technical reason for the discrepancy is the vertex distance—the space between the back surface of the corrective lens and the front surface of the cornea. For standard prescription glasses, this distance typically ranges between 12 and 14 millimeters. Contact lenses rest directly on the eye’s surface, making their vertex distance essentially zero.
Changing this distance affects the effective power of the lens, which is the strength required to focus light precisely onto the retina. Because the glasses sit further away, the light has more distance to travel before reaching the eye, altering how much the lens needs to bend the light to achieve the correct focus.
For nearsighted individuals (a minus prescription), moving the corrective lens closer to the eye, as contacts do, decreases the effective power needed for correction. Conversely, for farsighted individuals (a plus prescription), moving the lens closer increases the effective power. This difference is negligible for milder prescriptions. However, for corrections stronger than about \(\pm4.00\) diopters, the adjustment becomes significant and clinically necessary.
Differences in Visual Field and Peripheral Clarity
Beyond the mathematical adjustment of power, the physical placement of the lens creates a difference in the overall visual experience, particularly concerning the field of view. Contact lenses move seamlessly with the eye, covering the entire pupil and maintaining optical alignment across all viewing angles. This provides a natural, unobstructed field of vision that extends fully into the periphery.
Glasses are fixed in a frame, meaning the eye moves behind the lens when looking away from the center. Looking through the edges of the lens introduces aberrations, such as distortion and prismatic effects, which are more noticeable in stronger prescriptions. The frame itself also limits the peripheral view, which can make the central vision correction feel more intense in comparison.
The lack of peripheral distortions in contact lenses often makes the correction feel smoother or less abrupt than with glasses, even when the power is optically equivalent. This difference in visual quality and the absence of a frame barrier contributes to the feeling that the glasses are “stronger.” High-powered glasses can also subtly magnify or minimize the appearance of objects and the wearer’s eyes, an effect eliminated by contact lenses.
Why Contact Lens Prescriptions Require Conversion
Because of the vertex distance difference, eye care professionals cannot simply copy a glasses prescription for contact lenses. They must perform a specific calculation to ensure the same focal point is maintained on the retina. This conversion involves using the glasses prescription as a starting point and then mathematically adjusting the power to account for the zero vertex distance of the contact lens. The goal is to ensure that the vision clarity achieved by the contact lens is identical to that of the glasses, despite the different numbers on the written prescriptions.
In addition to the power conversion, contact lens prescriptions include unique specifications that do not appear on a glasses prescription, such as the base curve and diameter. The base curve determines how the lens fits the curvature of the cornea. A slight mismatch here can affect how the lens sits, influencing the perceived clarity or comfort. These fitting parameters, along with the intentional power adjustment, result in two distinct, non-interchangeable prescriptions designed to provide the same clear vision.