Eyeglass prescriptions (Rx) are a complex series of numbers and symbols that describe the specific optical corrections your eyes require. These notations, measured in units called diopters, are instructions for lens manufacturers to shape glass or plastic precisely. The combination of different numbers and signs—positive and negative—frequently leads to confusion. Understanding the basic components clarifies why your eyes may need a mix of corrective powers.
Defining Spherical and Cylindrical Lens Power
The first component is the Sphere (SPH), which corrects the eye’s overall focusing error, either nearsightedness or farsightedness. This power is necessary because the eye is either too long or too short, causing light to focus incorrectly on the retina. A spherical lens has the same curvature in all directions, providing a uniform power across the entire lens surface to shift the focal point back onto the retina.
The second component is the Cylinder (CYL), included only if you have astigmatism. Astigmatism is a common condition where the eye’s front surface is shaped more like a football, causing light to focus unevenly and creating blurriness. The cylindrical lens corrects this by adding power only along a specific meridian or axis. The spherical power handles the general refractive error, while the cylindrical power addresses the localized unevenness.
Decoding Plus and Minus Signs in Prescriptions
The signs preceding the spherical and cylindrical values dictate the type of lens required for correction. A negative sign (-) indicates a concave lens, which is thinner in the center and thicker at the edges. This lens causes light rays to diverge, pushing the focal point backward. Negative power is used to correct nearsightedness (myopia), where light focuses too far forward within the eye.
Conversely, a positive sign (+) indicates a convex lens, which is thicker in the center. This lens causes light rays to converge, pulling the focal point forward. Positive power is used to correct farsightedness (hyperopia), where light focuses behind the retina. Both signs specify the exact curvature and light-bending properties needed for clear vision.
The Optical Reason for Mixed Prescriptions
The question of whether a prescription can have a positive sphere and a negative cylinder is valid, and the answer is yes, this combination is optically sound and common. This mixed notation arises because the eye often needs two different corrections for its two primary, perpendicular meridians. For example, a person may have farsightedness (positive correction) combined with astigmatism, which is corrected using a negative power to balance the uneven focus.
This seemingly contradictory mix of signs is largely a matter of standardized notation, specifically the use of the minus cylinder form. In this common format, the cylinder value is always written as a negative number. This reflects the amount of power that must be subtracted from the spherical power along the astigmatic meridian.
A prescription written with a positive sphere and a negative cylinder is mathematically equivalent to a prescription written with a different sphere value and a positive cylinder. This equivalence is proven through a process called transposition, which involves a simple algebraic conversion. For instance, a prescription of +1.00 Sphere with -1.00 Cylinder can be transposed to 0.00 Sphere with +1.00 Cylinder. The mixed signs are not an optical contradiction but a standardized way to describe the combined power needed to correct the two distinct focusing problems simultaneously.