If you have ever received two different prescriptions—one for eyeglasses and one for contact lenses—you may have wondered if a mistake was made. This difference is not an error; it is a necessary adjustment based on the fundamental principles of optics. The human eye and its corrective lens form an optical system, and changing the physical position of the lens dramatically alters its required strength.
Understanding How Lens Power Works
Vision correction relies on the precise bending of light rays so they land perfectly on the retina at the back of the eye. The strength of a lens, or its ability to converge or diverge light, is measured in a unit called the Diopter (D). A higher Diopter value indicates a stronger lens that can bend light more sharply, while a lower value means a gentler correction.
A negative Diopter value, such as a -5.00 D prescription, corrects nearsightedness (myopia) by diverging light rays before they enter the eye. A positive Diopter value, like a +3.00 D prescription, corrects farsightedness (hyperopia) by converging light rays. The prescription must provide the exact amount of light bending needed to create a clear image, regardless of whether the physical lens is placed close or far from the eye.
The Critical Factor of Vertex Distance
The main reason your two prescriptions differ is a concept in optics known as vertex distance. Vertex distance is the physical space between the back surface of a corrective lens and the front surface of the cornea. For eyeglasses, this distance is typically around 12 to 14 millimeters, as the frame is held away from the face by the nose bridge and ear pieces. This small gap significantly influences the effective power of the lens.
In contrast, a contact lens sits directly on the tear film of the eye, making its vertex distance virtually zero. Since the goal is for the light to focus on the retina in the exact same spot, the lens power must be adjusted to account for the difference in physical separation. Moving the corrective lens closer to the eye changes the point at which the light rays converge or diverge, requiring a new power to maintain the same final focus. This difference in distance is the most important factor driving the conversion between glasses and contact lens prescriptions.
Adjusting Power for Proximity
Eye care professionals use vertex distance compensation to convert the glasses prescription to the necessary contact lens power. This conversion is required because the effective power of the lens changes with its proximity to the eye.
For someone with nearsightedness (a negative prescription), the contact lens must be optically weaker than the glasses lens. For example, a glasses prescription of -6.00 D might convert to a contact lens power of approximately -5.75 D.
Conversely, for a farsighted patient (a positive prescription), the contact lens must be optically stronger than the glasses lens. If a glasses prescription is +5.00 D, the contact lens equivalent may be around +5.25 D. This adjustment ensures that the light rays focus correctly on the retina from the new, closer position.
Practical Implications of Prescription Differences
For lower-strength prescriptions, the change in effective power is negligible. The vertex distance adjustment only becomes a significant factor when the glasses prescription is stronger than approximately ±4.00 D. At these higher powers, even a few millimeters of distance change can alter the required correction by 0.25 D or more, making the conversion a clinical necessity.
For patients with astigmatism, the conversion is more complex. While the spherical part of the prescription is adjusted for vertex distance, the cylindrical power may also require a slight change depending on its strength and axis.
The contact lens prescription also includes physical parameters not found on a glasses prescription, such as the base curve and diameter. These parameters are specific to fitting the lens directly onto the eye’s surface. Contact lenses often provide a wider field of view and better peripheral vision compared to glasses.