An eye exam provides a detailed assessment of your vision, leading to a prescription that guides the creation of corrective lenses. This prescription contains specific terms, often abbreviated, that describe the precise optical power needed to sharpen your sight. Understanding these terms can demystify your eyeglass prescription and offer insight into your unique visual needs.
Understanding Sphere
The term “sphere,” abbreviated as SPH, quantifies the overall lens power needed to correct common refractive errors like nearsightedness or farsightedness. This measurement is uniform across all lens meridians. Lens power is measured in diopters (D).
A negative sign, such as -2.00 D, indicates nearsightedness (myopia), where distant objects appear blurry. Conversely, a positive sign, like +1.50 D, signifies farsightedness (hyperopia), where close-up objects may seem blurry. The further the number is from zero, whether positive or negative, the stronger the lens correction needed.
Understanding Cylinder
The “cylinder,” abbreviated as CYL, addresses astigmatism. This condition occurs when the eye’s cornea or lens has an irregular, non-spherical shape, like a football. This irregular curvature causes light to focus unevenly on the retina, leading to distorted or blurred vision.
The cylinder value indicates the lens power needed to correct this unevenness. A cylinder value on your prescription means you have astigmatism requiring correction. If this column is blank or marked “SPH” or “DS,” you likely have no astigmatism or it is minor enough not to require specific correction.
Understanding Axis
The “axis” value works in conjunction with the cylinder to correct astigmatism. While the cylinder specifies the amount of power needed, the axis indicates the precise orientation of that corrective power on the lens. This measurement is given in degrees, ranging from 1 to 180.
The axis tells the lens manufacturer where to position the cylindrical correction to align with your eye’s irregular curvature. For instance, a 90-degree axis indicates a vertical orientation, while 180 degrees signifies a horizontal one. An axis value is only present on a prescription if there is a cylinder value.
Decoding Your Prescription
An eyeglass prescription organizes these values for each eye, using “OD” for the right eye (oculus dexter) and “OS” for the left eye (oculus sinister). You will see columns labeled SPH (Sphere), CYL (Cylinder), and AXIS, along with rows for OD and OS. These three numbers work together to create a lens that precisely addresses your individual vision needs.
For example, a prescription might read: OD -2.50 -0.75 x 170, OS -2.00 -1.00 x 10. This indicates that for the right eye (OD), there is -2.50 diopters of nearsightedness, -0.75 diopters of astigmatism, with correction oriented at 170 degrees. For the left eye (OS), there is -2.00 diopters of nearsightedness, -1.00 diopters of astigmatism, oriented at 10 degrees. This combination of sphere, cylinder, and axis ensures the lenses provide the clearest possible vision for each eye.