Normal Eye Axis: How to Read Your Prescription
September 8, 2022 • 5:21 PM
You’ve just completed your eye exam and gave yourself a pat on the back for doing so — kudos to you! Your optometrist then hands you a prescription slip. Squinting at the letters and numbers on the paper, you have no idea what they mean. The only eye-related terminology you know is “20/20 vision,” which doesn’t apply to you anymore since your prescription means you need corrective eyewear.
What’s more, the optometrist just told you that you have astigmatism and goes into a long-winded spiel about “normal eye axis” and “cylinder power.” You politely nod your head, but internally, you feel more confused than ever.
Don’t worry, you — and your peepers — are in good hands, as we’re about to spill the beans on “normal eye axis” and other terms typically found in eyecare prescriptions.
What Is the Eye Axis?
Before we talk about what counts as a normal eye axis, let’s first take a look at what the eye axis actually is.
For the official definition, a study in the Journal of Cataract and Refractive Surgery classified the axis as the lens meridian that doesn’t have any cylindrical power (you’ll learn what that is later on).
Dr. Emanuel Rosen, one of the pioneering editors of the journal, explained that “the cornea has no axes, only meridians ranging from zero to 180 degrees.” He further explained that the term “axis” only comes into play when describing the phoropter lens used to measure eye refraction during eye tests. He also likened the eye meridians to “the lines of longitude on a globe.”
That being said, Dr. Rosen acknowledged that most optometrists, opticians, and eye doctors tend to use “axis” instead of “meridians,” which birthed the term “eye axis.”
In human-speak, the eye axis refers to the location of astigmatism on your cornea. It's a value between 0 and 180 degrees. An axis value of 90 refers to the eye’s vertical meridian (from top to bottom). Meanwhile, a value of 180 corresponds to its horizontal meridian (from side to side).
The axis number doesn’t tell you how much astigmatism your eyes have — only how irregularly shaped your cornea is. As you may have heard, astigmatism is a refractive error in which the cornea doesn't have the ideal spherical curvature. Instead, the cornea is shaped like an egg. Some eyecare professionals even liken it to a rugby ball. This makes it difficult for light to land properly on the retina that's located at the back of the eyeball. As a result, people with astigmatism usually have blurry vision.
Cylinder Power With Regards to the Eye Axis
To find out how much astigmatism your eyes have, you’ll need to look at the cylinder power (CYL). This is the amount of lens power for astigmatism. The axis value is then used to guide the placement of the cylindrical power in the eyeglass lenses.
The cylinder power may also have a minus or plus sign. When you see a minus sign next to “CYL,” it means you have nearsighted astigmatism. Similarly, a plus sign next to your CYL value indicates farsighted astigmatism.
Take note that the axis number and cylinder power are only used in astigmatism prescriptions to indicate the location and the extent of your astigmatism. This will highlight to your optician whether you have a normal eye axis or not.
It’s possible to have a CYL value for only one eye, meaning that you only need astigmatism correction for that eye. If you don’t have astigmatism, your optometrist will leave blanks in the columns for the axis and cylinder power.
What Is a Normal Eye Axis?
A normal eye axis means that you don’t have astigmatism. Or at least, the astigmatism in your eyes isn’t significant enough to warrant corrective eyewear.
According to the American Academy of Ophthalmology (AAO), most people have “between 0.5 to 0.75 diopters of astigmatism.” (For the record, a diopter is the unit of measurement for the focusing power of corrective eyewear.) It’s only when your astigmatism measurement is 1.5 diopters or more that you need to wear eyeglasses or contact lenses. On the other hand, an astigmatism measurement of zero diopters means you don’t have this refractive error.
It’s also interesting to note that a normal eye axis, or even normal visual acuity (how clear your vision is), doesn’t necessarily mean perfect vision. Some people with 20/20 visual acuity may have color blindness or slow back-and-forth eye movements for visual tracking. As the AAO explains, the maximum visual acuity for humans can be “even finer at 20/16 to 20/12.”
Other Eyeglass Prescription Abbreviations You May Want to Know
Besides the axis and cylinder power, there are other abbreviations commonly found on eyeglass prescriptions. They represent other vision features like the right eye and left eye, as well as common visual problems — think myopia (nearsightedness), hyperopia (farsightedness), and presbyopia (age-related farsightedness). The values in these columns then tell the optometrist how much corrective power your new eyewear needs for each condition (if applicable), on top of whether you have a normal eye axis or not.
Let’s go through the commonly seen terms on an eyeglass prescription:
Oculus dexter (OD): This is the Latin phrase for the right eye, although some prescriptions use RE instead.
Oculus sinister (OS): This is the Latin phrase for the left eye, although some prescriptions use LE instead.
Oculus uterque (OU): This is the Latin phrase for both eyes.
Sphere power (SPH): This is the amount of lens power needed for vision correction measured in diopters (D). The higher the number, the stronger the lens power required to correct your vision. A minus sign indicates nearsightedness. Meanwhile, a plus sign or lack of it represents farsightedness. For example, +3.00 D represents three diopters of farsightedness.
Additional lens power (ADD): This refers to additional lens power if needed. It’s usually used for bifocals and multifocals to address age-related farsightedness. This allows the individual to stick with only one pair of eyeglasses instead of needing one for close vision and another for far vision. The sign used is always positive to indicate farsightedness.
Prism: Some prescriptions may have a column for the prism value to correct double vision (seeing two images of the same object).
Pupillary distance: This refers to the distance between the center of the eyes and is only needed for those wearing glasses. It’s usually measured in millimeters (mm), and helps the manufacturer ensure that the center of each eyeglass lens corresponds directly to the center of each pupil.
The prescription for contact lenses is slightly different from the one for eyeglasses. For one thing, contact lenses don’t need information on the pupillary distance. That said, contact lens prescriptions typically contain information for:
Base curve: How steep or flat your cornea is, which determines the size of your contact lenses.
Diameter: The length of your contact lenses to ensure they’re large enough to cover your cornea.
Contact lens prescriptions also include details about the brand, issue date, and expiry date.
Get Astigmatism Lenses for Your Eye Axis at Pair Eyewear
A normal eye axis simply refers to not having astigmatism. This doesn’t necessarily mean perfect vision, as it’s possible to be nearsighted or farsighted without astigmatism. Any axis value that’s above the normal range means that you likely need corrective eyewear for this refractive error and perhaps other vision problems, like myopia, depending on what’s on your prescription slip.
The good news is, you can get glasses for astigmatism and other vision problems at Pair Eyewear for as low as $60. We also offer progressive lenses and blue light-filtering lenses as add-ons to any of our frames. Shop the various fun collections to find your favorite pair(s) today!