INTERPRETING YOUR Rx

Optical prescriptions can be confusing, especially to the first-time spectacle wearer. Eye care professionals use lingo, abbreviations and numbers rooted in various areas of study, including Latin and physics. So when you think that we’re speaking a different language when we describe things, sometimes we actually do! But here, I’m going to try and lay things out into simpler terms so that you can better understand what all of the letters and numbers on your prescription mean. Everyone’s prescription is slightly different and some include components that others don’t, so bare with me as we try to hit all of the important points about interpreting spectacle prescriptions.

The Basics

The numbers in your prescription represent the power of the lenses required to correct your refractive error (i.e. myopia, hyperopia and/or astigmatism). Lens power is measured in units of dioptres (D) and prescribed in increments of 0.25D. The higher the dioptric power of a lens, the greater its ability to refract or bend incoming light.

Abbreviations are used to denote the right and left eye prescriptions:

OD = Oculus Dextrus, Latin for right eye.
OS = Oculus Sinister, Latin for left eye.

Scenario #1: No Astigmatism 

If your prescription doesn’t include astigmatism correction, only the first column will include a number. The second and third columns will either be left blank, crossed out or read “sph“, indicating that the lens is spherical.

SPH/sph = the sphere value, represents the refractive power of the lens required to correct the refractive error, in dioptres. The sphere value is preceded by a plus or minus sign:

+/Positive Lens Power = corrects for hyperopia or farsightedness
-/Negative Lens Power = corrects for myopia or nearsightedness

Scenario #2: Astigmatism 

Many individuals have at least some astigmatism included in their spectacle prescription. In an eye with astigmatism, the main refractive components, the cornea and lens, are not completely spherical. Rather than being shaped like a basketball or soccer ball, the cornea and/or lens are oblong or football-shaped, with two curvatures. The flatter curve is weaker than the steeper curve and the difference in refractive power between the two curvatures represents the amount of astigmatism. Therefore, a lens with two curves, called a spherocylindrical lens, is required to correct astigmatism. A direction must also be specified such that in total, there are three elements to a prescription for astigmatism:

sph/SPH = the sphere value, represents the power of the flattest curve in dioptres. This curve may have positive or negative power depending on whether that curve is myopic (nearsighted, requiring negative power) or hyperopic (farsighted, requiring positive power).

cyl/CYL = the cylinder value, represents the amount of astigmatism in dioptres. Optometrists in Canada always use negative cylinder power

axis/AXIS = the axis value, indicates the direction of the flattest curve in degrees. The axis value is anywhere between 0 and 180 degrees. 90 corresponds to a vertical orientation, while  180 is horizontal. These orientations can be likened to superimposing a protractor scale on the front surface of the eye (see below). In the example prescription above, the axes in the right and left eyes are 175 and 005, respectively. While this may seem like a large asymmetry between the eyes, it only represents a difference of 10 degrees because a change in axis beyond 180 degrees resets at 0 and changes incrementally from there.

Scenario #3: Reading Addition

Individuals with presbyopia or accommodative dysfunction will have different prescriptions for near and distance vision. Unless otherwise specifies, the numbers in the sphere, cylinder and axis columns generally represent the distance Rx.


Add
 = the reading addition, or add, is the specific amount of magnifying power added overtop of the distance sphere value to achieve the near Rx.

In the example above, the distance prescription in the right eye is +1.75D and +2.50 in the left eye. The near prescription is determined by adding 2.00D to each eye. Therefore, the near prescription in the right eye is +3.75D and +4.50 in the left eye.

 

Additional Points:

  • It is possible (and common!) for one eye to have astigmatism but not the other.
  • Slight changes in your astigmatism between prescriptions are normal. Remember that the smallest differences your optometrist shows you during subjective testing involve 0.25 dioptre changes. This is barely noticeable to most individuals and depending on the day, tester, testing conditions and instructions, may impact vision in different ways. Don’t worry if you go from having 0.25 or 0.50D of astigmatism in your Rx one year to not having any the next time you have your eyes tested (or vice versa). While it is possible that your eyes are actually changing, astigmatism is generally quite stable during the adult years
  • The add power is always the same for both eyes and is often recorded as a single value.
  • The add always has a positive value, even if it is not preceded by a plus sign.