How To Read Your Prescription

So you've been to see your optometrist and you've left with a little piece of paper. Congratulations! You have a prescription for eyeglasses. For some, this is a regular occurrence - you've been doing this since you were a child. For others, this is your first one and it is hard to understand. When you look at your prescription, you might see numbers listed under the headings of "OS: and "OD". They are Latin abbreviations: "OS" (oculus sinister) means the left eye, "OD" (oculus dextrus) means the right eye. You might on occasion, see a notation for OU, which means something involving both eyes. At other times, the prescription headings might just say "L" for left, "R" and "BOTH" referring to both eyes.

The numbers lives on a scale with 0 being "ideal". In general, the further away from zero the number on your prescription, the more challenged your eyesight and the more vision you need.  A "+" sign in front of the number means you are farsighted, and a "-" sign means you are nearsighted. These numbers are the unit used to measure the correction, or focusing power, of the lens your eye requires. This is known as diopter. Diopter is often abbreviated "D" and are in increments of 0.25.


For example, if your prescription says -1.00, you have one diopter of nearsightedness. This is a fairly mild amount of nearsightedness. If you are -4.25, that means you have 4 and 1/4 diopters of nearsightedness. This is more nearsighted than -1.00, and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness and +5 would be more.

For people who have astigmatism, there will be three numbers in your prescription. The general form for writing these numbers is S (or SPH)C (or CYL) x Axis.  A fourth number for each eye might say ADD.

The S (or SPH) refers to the "spherical" portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above.

The C (or CYL) 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 football than a basketball.

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 stands for Addition. It stands for the additional correction that you need for reading. It is used in bifocal glasses, reading glasses, or varifocal glasses. It represents the additional power over the distance prescription.  We currently don't offer bi-focal or progressive lenses but may if the demand arises.

Here are two examples of what a prescriptions for an eye with astigmatism could look like:

S = -2.00, C= +1.50, AXIS = x 180

This prescription means that the person has 2 diopters of nearsightedness with 1.5 diopters of astigmatism and an axis of 180 degrees.

S = +3.50, C = +3.00, AXIS = x 45

This prescription means that the person has 3.5 diopters of farsightedness, 3 diopters of astigmatism and an axis of 45 degrees.

Pupillary Distance (PD)

Pupillary distance (PD) measures the distance between the centers of your pupils. This measurement is used to determine where you look through the lens of your glasses and should be as accurate as possible. The average adult’s PD is between 54-74 mm; kids' are between 43-58 mm. Your eye doctor will usually measure your PD during an eye exam. However, if it was not given to you, the below 5 steps will help you measure it yourself.

How to Measure Your PD

  1. Stand 8 in. away from a mirror.
  2. Hold a ruler against your brow.
  3. Close your right eye then align the ruler’s 0 mm with the center of your left pupil.
  4. Look straight then close your left eye and open your right eye.
  5. The mm line that lines up to the center of your right pupil is your PD.

 How to Measure Pupillary Distance (PD) | EyeBuyDirect

For more information, contact an Optometrist in your area.