The question of whether to wear glasses when only one eye has a refractive error addresses a specific condition known as anisometropia. Anisometropia means the two eyes have significantly different refractive powers, such as one eye being nearsighted or farsighted while the other is near-perfect. A substantial difference in prescription between the two eyes requires professional intervention because the visual system relies on both eyes working in precise coordination. Leaving the uncorrected eye can disrupt the delicate balance required for comfortable and efficient sight.
Why Correcting One Eye is Crucial for Binocular Vision
The reason for correcting a refractive error in a single eye is maintaining proper binocular vision, the brain’s ability to create a single, three-dimensional image from the input of both eyes. When one eye is blurry and the other is clear, the brain receives two mismatched images, leading to aniseikonia, or a difference in image size. The brain struggles to fuse these unequal images into one coherent picture, which causes visual discomfort.
This image disparity compromises stereopsis, the depth perception that relies on the slightly different angle of view from each eye. A clear, balanced input from both eyes is necessary for the brain to calculate spatial relationships accurately. When the input is unbalanced, the brain works harder, leading to eye strain. Correcting the refractive error in the weaker eye provides the clear, equally sized image needed for the two eyes to work together as a single functional unit.
Specific Conditions that Require Unilateral Correction
The cause of needing correction in only one eye is most often anisometropia, defined as a difference in refractive power of one diopter or more between the two eyes. This condition involves one eye being more myopic (nearsighted) or hyperopic (farsighted) than the other. Anisometropia is the leading cause of a related condition called amblyopia.
Amblyopia, or “lazy eye,” occurs when the brain suppresses the blurry image from the weaker eye during the period of visual development in early childhood. If the difference in prescription is left uncorrected, the brain learns to ignore the input from the blurry eye permanently. An uncorrected difference of 1.5 diopters or more, particularly in farsightedness, increases the risk of visual deterioration. For high refractive errors, a difference greater than 3.0 diopters is considered clinically significant and requires correction to prevent long-term vision loss.
What Happens If the Bad Eye is Left Uncorrected
Ignoring a refractive error in one eye can lead to immediate and long-term negative outcomes. The effort required to manage the mismatched images often results in asthenopia, the medical term for eye strain, which manifests as headaches and visual fatigue. People may also experience disorientation or dizziness as their visual system struggles to maintain focus and depth perception.
Over time, the consequence is the development of visual suppression, where the brain switches off the input from the weaker eye to eliminate confusion. This suppression leads to a reduction or loss of stereopsis, meaning the person loses the ability to perceive depth accurately. If this pattern begins in childhood, it causes amblyopia, resulting in permanently reduced visual acuity that cannot be fully corrected later in life. Even in adults, uncorrected anisometropia can create discomfort and limit the quality of binocular function.
Practical Solutions for Correcting Only One Eye
Correcting a unilateral refractive error involves providing the necessary power to the weaker eye while ensuring the two eyes can work together comfortably. Eyeglasses are a common solution, and the lens for the stronger eye is often a plano lens (no corrective power) or a minimal prescription for balance. However, in cases of high anisometropia, typically above 3.0 diopters, standard spectacle lenses can induce aniseikonia, making the image size difference too large for the brain to fuse.
In higher-difference cases, contact lenses are the preferred choice because they sit directly on the cornea, minimizing the magnification or minification effects that cause image size disparity. Contact lenses reduce aniseikonia and allow for better binocular fusion and depth perception. For patients with high, stable differences, refractive surgery is sometimes considered as a permanent way to equalize the refractive power between the two eyes, reducing strong corrective lenses.