Eyeglass prescriptions often appear as a series of numbers and abbreviations that can seem indecipherable to the average person. Among these values, the “cylinder” component is particularly confusing, especially when accompanied by a plus sign. This notation is a precise instruction to the lens laboratory, detailing a specific type of vision correction needed for clarity. Understanding this element clarifies how your lenses are customized to improve your sight.
Understanding Astigmatism
The presence of a cylinder value in a prescription indicates the eye has a refractive error called astigmatism. This condition is caused by an irregular curvature of the eye’s cornea or, less commonly, the lens. Instead of being perfectly spherical, an astigmatic eye is shaped more like an American football, featuring different degrees of curvature in different directions, or meridians.
This uneven shape prevents light from focusing correctly at a single point on the retina. Instead, light focuses at multiple points, resulting in vision that is blurry, stretched, or distorted at all distances. Astigmatism frequently occurs alongside other refractive errors like nearsightedness or farsightedness, which is why a prescription usually contains multiple components.
Decoding the Cylinder (CYL) Value
The numerical value listed under the “CYL” column specifies the magnitude of the correction required for astigmatism. This number is measured in diopters (D) and indicates the strength of the cylindrical lens needed to neutralize the eye’s uneven curvature. The higher this number, the greater the degree of astigmatism and the stronger the corrective power the lens must contain.
Accompanying the cylinder power is a value called the “Axis,” measured in degrees from 1 to 180. The Axis does not represent the strength of the prescription but rather the precise orientation or angle at which the astigmatism is located. Since astigmatism requires correction in a specific direction, the cylindrical lens must be aligned to this exact angle to be effective.
The Significance of the Plus Sign (+)
The plus sign in a plus cylinder prescription directly addresses the type of lens power convention used by the eye care professional. It is one of two primary ways to write a cylindrical correction, the other being the minus cylinder notation. The choice between plus (+) or minus (-) cylinder is often a matter of the prescribing doctor’s preference.
A prescription written in plus cylinder format indicates that the lens power is being calculated using a specific optical method. Importantly, a plus cylinder prescription can describe the exact same physical lens as one written in minus cylinder notation. Because manufacturing equipment typically uses the minus cylinder format, an optician will often mathematically “transpose” a plus cylinder prescription before ordering the lenses.
When transposition occurs, the sphere, cylinder, and axis values all change, but the final corrective lens remains optically identical. For the person wearing the glasses, the sign itself does not alter the vision correction they receive; it is simply a standardized way of communicating the lens design. The plus sign primarily informs the optician and the lab about the calculation method.
How Cylindrical Lenses Correct Vision
The lens used to correct astigmatism is called a cylindrical lens, which functions differently from a spherical lens designed for simple nearsightedness or farsightedness. Unlike a spherical lens, which has a uniform curve across its surface, a cylindrical lens has two distinct curvatures. This unique shape compensates for the eye’s irregular surface.
The lens is designed to have zero optical power along one meridian, corresponding to the Axis number on the prescription. Perpendicular to this, the lens has its maximum corrective power, which is the value listed under the CYL number. By introducing this counter-distortion, the lens precisely neutralizes the uneven focusing caused by astigmatism.
This specialized bending of light ensures that all incoming light rays are focused correctly onto a single, sharp point on the retina. The cylindrical lens essentially reshapes the incoming light wave, correcting the blurring that would otherwise occur. This precise optical counterbalance transforms distorted vision into clear, comfortable sight.