An eyeglass prescription for a person without an astigmatism usually has three headings, according to WebMD: "OS" refers to the left eye, "OD" refers to the right eye, and "OU" refers to both eyes. The numbers are diopters, the units of focusing power needed to correct vision. The higher the diopter number, the more vision correction the patient needs. A plus sign indicates farsightedness, while a minus sign indicates nearsightedness.
Some prescriptions might have the diopters abbreviated as "D" after the number, explains WebMD. For example, an eyeglass prescription that reads "OS -4.00 / OD -3.00" means that the person has a high degree of nearsightedness in both eyes, but his right eye needs less vision correction than his left eye. If the prescription reads "OU +1.50," it says that the person is equally farsighted in both eyes and that he requires 1.5 diopters of focusing power in each lens of his eyeglasses.
A person with an astigmatism receives an eyeglass prescription with three numbers; the first number is the sphere, which indicates either nearsightedness or farsightedness in the same way as it does for a person without an astigmatism, reports WebMD. The second number is the cylinder, or the astigmatism itself, in either a positive or a negative number. A higher number indicates a higher degree of astigmatism. The third number, which may be between zero and 180, is a measure of degrees, showing where on the curvature of the eye the astigmatism affects the patient's vision.