You met with your optometrist or ophthalmologist and, after what was probably an uncomfortable eye examination, you were handed your prescription along with a short description of the overall state of your eyesight - maybe you’re nearsighted or farsighted, or perhaps you have astigmatism.
Either way, now you have your prescription in hand and you’re ready to make a purchase to correct your eyesight, but what exactly do all these numbers mean? Understanding your prescription is key to making the right decision for you in terms of contact lens choice, so let the decryption begin!
Breaking it down abbreviation by abbreviation:
OD and OS : These are abbreviations for Oculus Dexter and Oculus Sinister, which are the Latin terms for right eye and left eye, respectively. To know the details of your prescription for each individual eye, simply follow the line. Your prescription may also have a column labeled "OU." This is the abbreviation for the Latin term, Oculus Uterque, which means "both eyes."
Sphere (SPH): This indicates the amount of lens power, measured in diopters (D), prescribed to correct nearsightedness or farsightedness. If the number appearing under this heading has a minus sign (–), you are nearsighted. If the number has a plus sign (+) or is not preceded by a plus sign or a minus sign, you are farsighted.
Cylinder (CYL): This indicates the amount of lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not necessary to correct it with your contact lenses. The number in the cylinder column may be preceded by a minus sign (-), in order to correct a nearsighted astigmatism, or a plus sign (+), for the correction of a farsighted astigmatism. Cylinder power always follows sphere power in a prescription to correct eyesight.
Axis: This describes the angle of the astigmatism indicated in the previous column. The axis is defined using a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian. If the (CYL) column is left blank due to a lack of astigmatism, then the Axis column will also be blank.
Prism: This is the amount of prismatic power, measured in prism diopters (D), prescribed to compensate for eye alignment problems. Only a small percentage of prescriptions include prism.
Add: This is the added magnifying power on top of your distance prescription to correct Presbyopia. The number appearing in this section of the prescription will always be a positive power, even if it is not preceded by a plus sign.