The experience of seeing clearly through eyeglasses only to have vision blur with contact lenses is a common source of frustration. This discrepancy suggests the issue is not with the underlying refractive error, but rather with the way the corrective lens interacts with the eye. Unlike glasses, which are fixed in a frame, contact lenses are dynamic medical devices that rest directly on the eye’s surface, making them susceptible to physical and environmental factors. The solution to this blur often lies in a precise re-evaluation of the lens’s physical parameters, the conversion of the prescription, or the maintenance routine.
The Optical Gap: Distance and Magnification
The most fundamental difference between glasses and contacts is the physical distance from the eye, a measurement known as vertex distance. Eyeglasses typically sit 12 to 14 millimeters away from the cornea, creating an optical system where the lens power is measured at that specific distance. Contact lenses, however, eliminate this distance by resting directly on the eye’s surface. This change in position affects the lens’s effective power.
For individuals with low prescriptions, the difference in power is negligible, and the same prescription can often be used for both glasses and contacts. The vertex distance becomes optically significant for higher prescriptions, generally those stronger than \(\pm 4.00\) diopters. For example, a spectacle prescription of \(-6.00\) diopters for nearsightedness will require a lower power contact lens, approximately \(-5.50\) diopters, to provide the same clear vision at the corneal plane. If this calculated conversion is not performed correctly, the contact lens will deliver an under- or over-correction, resulting in blurry vision.
Fit and Prescription Mismatch
A contact lens must conform precisely to the unique shape of the eye, and two specific measurements govern this physical fit: the Base Curve (BC) and the Diameter (DIA). The Base Curve dictates the curvature of the back surface of the lens, which is designed to match the curvature of the cornea. Soft contact lenses typically have a BC between 8.0 and 9.5 millimeters.
If the Base Curve is too flat, the lens will move excessively on the eye, often leading to temporary blur after a blink as the lens fails to center properly. Conversely, a lens with a Base Curve that is too steep will fit too tightly, restricting tear flow and potentially causing discomfort and distortion.
The Diameter, the overall width of the lens (usually between 13.0 and 14.5 millimeters), also plays a role in centering and stability. An incorrect Diameter can cause the lens to shift or not cover the cornea adequately, leading to poor visual acuity.
Lens Surface and Environmental Factors
Blurry vision that develops or fluctuates throughout the day is often linked to the lens surface or the eye’s immediate environment. Contact lenses rest on the tear film, and any disruption to this film, such as with Dry Eye Syndrome, can lead to fluctuating clarity. The lens material itself can exacerbate dryness, causing the lens to lose moisture and shape, which results in a hazy, uncomfortable sensation.
The lens surface collects biological materials found in tears, such as proteins and lipids, creating deposits over time. These deposits manifest as hazy, cloudy vision and can reduce the lens’s clarity and breathability. Improper cleaning or wearing lenses past their recommended replacement schedule significantly accelerates this buildup. Physical damage, such as a small tear or chip, can also distort the image and cause vision to blur.
Astigmatism and Rotational Stability
The correction of astigmatism with contact lenses involves a unique challenge that is not a factor with glasses. Astigmatism correction requires a specific lens type, known as a toric lens, which has different refractive powers along two perpendicular axes. For the toric lens to provide clear vision, these axes must align perfectly with the astigmatism in the eye.
Toric lenses are engineered with stabilization features, such as prism ballast or thin-zone designs, intended to keep them from rotating on the eye during a blink. However, if the lens rotates even slightly, the corrective axis shifts out of alignment, causing vision to become blurry. A rotation of just 10 degrees can leave a significant amount of the astigmatism uncorrected, leading to substantial visual blur. This instability explains why vision may be sharp one moment and blurry the next, unlike a spectacle lens, which is fixed in a frame and provides stable correction.