No doubt you’ve had your sight test reminder, you book an appointment and walk away with a new prescription…what does it all mean?
SPH or Sphere. 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). The C refers to the “cylinder” or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that you have. The bigger this number, the more astigmatism you have. Astigmatism most often is caused by a cornea that is shaped more like a rugby ball than a football.
Axis. The Axis is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism. It is not enough to specify how much astigmatism there is; you have to know where the difference in curvature is taking place.
Add. This is your reading addition and relates to the amount of additional correction needed to focus at close distances.
Prism/Base. Sometimes, you may have a muscle imbalance in your eye, so your optician will prescribe a prism and a prism base. The prism is usually written in fractions (for example 1 ½) and the prism base shows the direction of the prism in the lens (for example, IN, OUT, UP or DOWN). This is fairly uncommon so the boxes are normally empty.
You have your prescription, how do you know if you need single vision, bi-focals or vari focals?
Single Vision. Single Vision prescriptions are for patients who have trouble seeing either near or far (but not both). Your prescription will have no value in the ‘Add’ column.
Bi-focal / Varifocal Vision. Bifocal / Varifocal prescriptions are for patients who have trouble seeing both near and far. You will have a value in the ‘Add’ column.