When ordering eyeglasses for vision correction, selecting style and color is just the beginning. You also need to provide your prescription numbers, which dictate how the lenses of your new eyewear will correct your vision. But what do these numbers mean?
The abbreviations OS and OD stand for Oculus Sinister and Oculus Dextrus, Latin for your left and right eyes, respectively.
The letter S or SPH stands for "spherical," referring to the correction needed for farsightedness or nearsightedness. The number next to it indicates the amount of correction required.
A zero indicates perfect vision. Positive numbers indicate farsightedness correction, while negative numbers indicate the correction levels for nearsightedness.
For astigmatism, you will also see a positive or negative number next to "C," which stands for "cylinder." Astigmatism occurs when the outer layer of the eye, the Cornea, is more oval than spherical.
However, astigmatism requires another number next to "A" or "Axis," which represents the curvature difference, or the degrees of rotation of the Cornea's oval shape. This number should fall between 0 and 180.