Glasses are widely recognized as a common tool for improving vision, though the specific term astigmatism often causes confusion. Glasses function as external lenses that adjust how light enters the eye, ensuring it focuses correctly on the retina. Understanding what astigmatism is and how it relates to other vision problems is essential. This article clarifies the role of astigmatism within the broader need for corrective eyewear.
The Answer: Refractive Errors That Require Glasses
The short answer to whether everyone with glasses has astigmatism is no, because multiple vision problems necessitate the use of corrective lenses. Eye care professionals classify these issues as refractive errors, conditions where the eye cannot properly bend light to create a sharp image. The three main types of refractive errors that commonly require correction are myopia, hyperopia, and astigmatism.
Myopia (nearsightedness) occurs when the eye is too long or the cornea is too steeply curved, causing light to focus in front of the retina. Conversely, hyperopia (farsightedness) happens when the eye is too short or the cornea is too flat, resulting in light focusing behind the retina. Both are considered spherical errors because the required lens correction is uniform across all meridians of the lens.
Understanding the Mechanics of Astigmatism
Astigmatism is characterized by an irregular curvature of the cornea, the clear front surface of the eye, or sometimes the lens inside the eye. A normal cornea is shaped like a perfect sphere, with an even curve in all directions. In an eye with astigmatism, the surface is curved more like a football, where the curve is steeper in one direction than the other.
This asymmetry causes light entering the eye to bend unevenly, preventing it from converging onto a single focal point on the retina. Instead, the light focuses on multiple points, resulting in vision that is blurry or distorted at all distances, both near and far. This condition can occur alone or in combination with either myopia or hyperopia.
The uneven focusing of light often leads to noticeable symptoms beyond just blurriness. People with astigmatism frequently experience eye strain, especially after focusing for extended periods. Headaches, difficulty seeing fine details, and streaking of lights at night are also commonly reported issues. These symptoms signal the need for an eye examination.
How Astigmatism is Measured and Corrected
Correcting astigmatism requires a specific type of lens that can compensate for the irregular shape of the eye’s surface. Eye doctors measure the degree and orientation of astigmatism during an eye exam and translate these findings into specific values on a prescription. The measurement for astigmatism is provided in diopters, which is the unit of optical power.
Two specific values are used on a prescription to address astigmatism: Cylinder (CYL) and Axis. The Cylinder value indicates the severity of the astigmatism, or how much the eye deviates from a spherical shape. A higher numerical value in the cylinder column signifies a greater degree of irregularity that needs correction.
The Axis value is a number between 1 and 180 degrees that specifies the exact orientation of the steepest curvature on the cornea. This measurement is necessary because the lens must be ground with its own steeper curve aligned precisely to counteract the eye’s irregularity. The resulting corrective lenses contain a cylindrical power that has different strengths across different meridians, which precisely bends the light to achieve a single, clear focal point on the retina.