How to Read Your Eyeglass Prescription
So you’ve been handed a piece of paper by your optometrist or ophthalmologist with your eyeglass or contact lens prescription. Perhaps you’ve been given these pieces of paper for many years. It’s full of mysterious abbreviations, terms and numbers, and you’re curious…. Just what does it all mean?
Understanding your eyeglass prescription takes a bit of work, but it can be interesting stuff. Many people are intrigued about what the magic formula their eye doctor has come up with actually says about their eyes.
Your prescription is all about correcting refractive error – an eye condition that affects millions of us all over the world. (Read Refractive Error Revealed to understand more about how refractive error works.)
Refractive error causes blur, and our glasses or contact lenses correct for this blur based on our own particular type of refractive error. (This is why if you try on someone else’s glasses, you usually see blur. What corrects another person’s refractive error is not what you need, so it makes your vision worse.)
Nearsighted people typically experience blur when looking at objects that are far away. Farsighted people typically see things well when far away, but encounter blur when looking at things close up.
What is a Diopter?
Let’s start with a standard unit of measurement on eyeglass prescriptions, called a diopter.
A diopter can be a negative number (which indicates nearsightedness and a lens that minimizes things). Or it can be a positive number (which indicates farsightedness and a lens that magnifies).
But what exactly is a diopter? A diopter simply indicates how powerful a lens is in order to properly focus light on a person’s retina, and it is defined as being “the inverse of a person’s focal length in metres.” Focal length is the distance you need to see an object clearly for a fine-detail task, for example to read a book.
So, a nearsighted person who needs a –1.00 diopter lens can see objects at one metre clearly, but anything farther is blurred. Someone with a –2.00 diopter measurement requires a lens that is twice as powerful – so they can only see objects up to a 1/2 metre away clearly. A –3.00 lens would mean the person can only see a distance of up to 1/3 of a metre clearly, and so on. Most nearsighted people are in the range of -1.50 to -7.00 diopters, which is considered mild to moderate.
On the other hand, a farsighted person who needs a +1.00 diopter lens can see objects at one metre clearly, but anything closer than that is blurred. A +2.00 lens indicates someone can see things at 1/2 metre and beyond clearly, but nothing closer.
Throw a Curve into the Equation
There’s more to the story though.
Many people also have astigmatism, which means that their nearsightedness or farsightedness is not the same in all directions. This is because their corneas are not perfectly round, but stretched irregularly, like a cylinder (or the back of a spoon).
Astigmatism makes vision blurry at any distance. A good analogy is an artist who rubs his thumb across a charcoal drawing, smearing (blurring) the image. The image looks distorted in a particular direction, rather than being distorted in all directions. Astigmatism does something similar to that charcoal smudge on top of the nearsightedness or farsightedness that we already have.
The other thing is, our two eyes can be very different in terms of refractive error. We may have astigmatism and some nearsightedness in one eye, and even more nearsightedness but no astigmatism in the other, for example. Rarely do people need exactly the same correction for both of their eyes.
When your eye doctor corrects for astigmatism, it is called a cylindrical correction. It is called cylindrical because it has no curve (or power) in one direction, and maximum power at a 90-degree angle to the original direction. (Correcting for nearsightedness or farsightedness is called a spherical correction, because it is correcting in all the directions equally.)
Your eye doctor has to determine what axis your astigmatism occurs on in your eye. To use our previous example, she would be trying to find out where is that charcoal smudge located, exactly?
Think of an eye, if you were to look at it straight on, as a clock face. At the nine o’clock position, this is considered zero degrees. The measurement continues up to 180 degrees, and goes counter-clockwise from the nine o’clock position. So, at the six o’clock position, you would be at 90 degrees, and at 3 o’clock, you would be at 180.
Measurements for astigmatism don’t need to go higher than 180. Why? Because just like that charcoal smudge, when you locate astigmatism in one direction on a 180 degree axis, you assume that it continues diagonally through the entire line of vision.
Putting It All Together
OK, let’s look at a sample eyeglass prescription, define some terms, and get the big picture. Note, however, that eyeglass prescriptions can vary in how they are written, so yours may not look exactly like this example.
- On the left, D.V. stands for “Distance Vision” and N.V. stands for “Near Vision.” D.V. is where your prescription would go. N.V. is used if you have glasses only for reading, or if you have bifocals, N.V. indicates the lower portion of your bifocal lens.
- O.D. stands for oculus dexter, Latin for “right eye.” (Your prescription may just say “right” or even RE.
- O.S. stands for oculus sinister, which is “left eye” in Latin.
- Spherical is where you find a measurement, in diopters, for nearsightedness or farsighteness.
- Cylindrical is where you find a measurement, in diopters, to correct for astigmatism.
- Axis tells where your astigmatism is located along the horizontal axis in your eye (described above).
- Add: If you have a column called Add in your prescription, this is for bifocals. (A measurement for the lower portion of your bifocals may appear here, instead of in a N.V. line. The “add” is usually additional diopters of strength that are added onto your usual prescription in the lower portion of the lens.)
In this example, this person has a –3.25 correction for nearsightedness in the right eye, and a +.5 correction for farsightedness in the left.
In the right eye, the person also has 130 degrees listed in the Axis column. This means that 130 degrees is the direction that the person’s vision is not distorted by astigmatism. So what this is saying is that at 130 degrees, there is a simple –3.25 correction for nearsightedness.
But, in the perpendicular direction, this person’s vision is distorted by astigmatism. The perpendicular of 130 degrees would be at 40 degrees (130 minus 90 degrees). So along the 40-degree axis, the person has to have an additional correction. How much correction? Add the numbers under Spherical and Cylindrical together. So in this case, the correction would be -3.5 in the 40-degree direction.
Now that you know how to “read” the right eye, the left eye should be easier. In the left eye, the person has a +.5 correction for farsightedness along the 80-degree axis. And at the perpendicular axis (80 degrees plus 90 degrees this time, to stay in the same 180 degree range), at 170 degrees, the person has a -.5 degree correction for nearsightedness.
That’s it! So now you can decipher your own prescription, and understand what it reveals about your eyes. It won’t win you any new friends at cocktail parties, but you will be a little bit more informed about your vision and how it changes over time.
And next time you are having an eye exam, you can ask, with authority, “Have my dioptic measurements changed this time, doctor?”