A prescription is a precise medical tool designed to correct refractive errors and achieve the clearest possible vision. Many people consider wearing contact lenses with a lower prescription, often to save money or use an older supply. Corrective lenses must match the specific optical needs of your eye. The lens power, measured in diopters, represents the degree of correction needed to focus light precisely onto the retina.
The Core Answer: Why Prescription Accuracy Matters
The immediate goal of any corrective lens is to achieve optimal visual acuity, often measured as 20/20 vision. Using a contact lens with a power weaker than prescribed means you are introducing a deficit in your vision correction. This under-correction results in a noticeable loss of clarity, making distant objects appear blurry or slightly indistinct.
Even a small difference, such as 0.25 or 0.50 diopters, can compromise visual function, especially during tasks demanding sharp focus. Activities like reading fine print or driving require your eye muscles to strain constantly to compensate for the blur. This continuous effort to sharpen an image that is not fully corrected leads to visual fatigue and discomfort.
Understanding the Difference Between Contact Lens and Eyeglass Prescriptions
A common source of confusion for patients is that their contact lens power is often slightly weaker than their eyeglass power. This difference is not an under-correction but a necessary adjustment based on the lens’s position relative to the eye. Eyeglass lenses sit approximately 12 to 14 millimeters away from the cornea, a measurement known as vertex distance.
Contact lenses, however, rest directly on the surface of the cornea. The change in distance alters how light travels through the lens to reach the eye’s focusing system, which changes the required power. For people who are nearsighted (minus power), moving the lens closer to the eye means less minus power is needed to achieve the same full correction.
The eye care professional calculates this conversion factor to ensure the contact lens power provides the precise vision correction as the glasses. This conversion is most significant for prescriptions higher than ±4.00 diopters, but it is standard practice for nearly all prescriptions. Therefore, a “weaker” contact lens power may actually be the correct, full prescription for your eye’s needs.
Health Consequences of Consistent Under-Correction
Wearing a consistently weaker contact lens than required forces the eyes to work harder, leading to a condition known as asthenopia, or chronic eye strain. The eyes’ focusing muscles, or ciliary muscles, continuously attempt to accommodate the remaining blur. This prolonged muscular tension often manifests as frequent, dull headaches, particularly after extended periods of visual work or driving.
The constant struggle to focus can also contribute to general fatigue because the visual system is overworking. For children and adolescents with nearsightedness, wearing under-corrected lenses is a particular concern. Some research suggests that consistent under-correction may potentially contribute to the progression of myopia. Following the exact prescribed power is important for maintaining ocular comfort and preventing these symptoms.