Vision correction often involves confusing terminology when understanding specific lens types. The question of whether bifocals are needed to correct astigmatism is common. This confusion arises because both address vision problems, but they are designed to solve entirely different issues. This article clarifies the distinct nature of astigmatism and the function of bifocal lenses, explaining why one does not require the other.
Understanding Astigmatism
Astigmatism is a refractive error, meaning it is a problem with how the eye focuses light. This condition is caused by an imperfect curvature of the eye’s front surface, the cornea, or sometimes the internal lens. In a normally shaped eye, the cornea is perfectly spherical, which allows light to focus evenly at a single point on the retina.
With astigmatism, the corneal surface is shaped more like a football, possessing a steeper curve in one direction and a flatter curve 90 degrees away. This irregularity causes incoming light rays to focus unevenly at multiple points. The result is vision that appears blurred or distorted at any distance. This visual problem is measured by a component in a prescription called the cylinder power, which quantifies the degree of the irregularity.
The Purpose of Bifocal Lenses
Bifocal lenses were developed to treat a condition called presbyopia, which is an age-related loss of the eye’s focusing ability. As a person ages, typically starting around age 40, the natural lens inside the eye gradually becomes less flexible. This reduced elasticity makes it difficult for the eye to change shape and focus on objects held close.
The design of a bifocal lens directly addresses this loss of near-focusing power. It contains two distinct optical powers within a single lens, usually separated by a visible line. The larger top portion of the lens is dedicated to distance vision correction, while the smaller segment at the bottom provides a stronger power for seeing things up close. This near-vision boost is indicated on a prescription as the “add power” and restores the ability to focus at reading distance.
Bifocals and Astigmatism: Addressing Two Separate Issues
The need for bifocals and the presence of astigmatism are two separate and independent vision issues. Astigmatism is a fixed problem of the eye’s shape, present regardless of age, requiring correction for distorted vision. Conversely, the need for bifocals is a progressive problem of the eye’s focusing mechanism that develops due to the natural aging process (presbyopia).
Astigmatism requires a specific cylindrical correction to counteract the uneven curvature, while presbyopia requires an “add power” for close-up focus. Bifocal lenses alone only provide the near and distance powers for presbyopia; they do not inherently contain the geometry necessary to correct astigmatism. However, eyeglass lenses are commonly customized to include both the cylindrical correction for astigmatism and the “add power” for presbyopia simultaneously.
Corrective Options Beyond Bifocals
The primary method for correcting astigmatism, whether or not a person needs bifocals, involves specialized optical components that compensate for the irregular eye shape. In glasses and contact lenses, this correction is achieved using a specific power called the cylinder, which neutralizes the uneven curve.
For contact lens wearers, the solution is the Toric lens. Toric lenses are specifically engineered with different refractive powers in their horizontal and vertical orientations, matching the two different curves of the astigmatic eye. These lenses must maintain a stable orientation on the eye to function correctly, often using features like weighted bases or thin zones to prevent rotation. For those who want to correct astigmatism, refractive surgery options like LASIK or Photorefractive Keratectomy (PRK) can reshape the cornea with a laser to create a more spherical surface.