When you leave your eye doctor's office and look at your prescription, it may seem like an unintelligible series of numbers and abbreviations. Luckily, what may seem like a complex code is easy to crack. The specific combinations of letters and numbers let your eyeglass manufacturer know not only if you're nearsighted or farsighted, but also if you have astigmatism or presbyopia.
This article will go behind the numbers to help you understand your prescription so you can be knowledgeable about your own eye health, track any changes in your vision, and talk comfortably with your eye care practitioner.
Read on for more about the specific abbreviations, letters, and numbers and what they mean when it comes to your eyewear prescription.
Distance: This is the first word you will see on the card. This term refers to what power of lens you will need to see distant objects clearly. Your ophthalmologist or optometrist determines this power by trying different lenses while you read an eye chart in the exam. The strength of lens correction you need may be different for each eye.
OD vs OS: These stand for right eye ("oculus dexter") and left eye ("oculus sinister") in Latin. You may just see RE for right eye and LE for left eye, as some doctors have moved away from the traditional Latin prescription terms. If you see OU, it stands for "oculus uterque," meaning both eyes are measured the same.
Sphere (SPH): This refers to the lens power you'll need to achieve 20/20 (or at least optimal) vision. A minus sign (-) means you are myopic, or nearsighted. A plus sign (+) means you are hyperopic, or farsighted. (If you see no + or - sign, it also means you are farsighted.) The higher your SPH number is, the more correction you will need.
Cylinder (CYL): This is used to denote the lens power you’ll need to correct astigmatism. If the cylinder field is blank, you don't have astigmatism, or you only have a very mild one that doesn't need correcting. Again, a minus sign (-) corresponds to nearsighted astigmatism, and plus sign (+) corresponds to farsighted astigmatism.
AXIS: The axis is a number between 1-180 indicating the meridian of the eye. If you imagine your eye as a circle and draw a horizontal line to halve that circle, the meridian is one-half. If you think back to high school geometry, you'll remember that "90" is the vertical line drawn straight up and down (the vertical meridian), while "180" is the horizontal line (the horizontal meridian).
If your prescription indicates that you need a cylindrical lens power (for astigmatism), then you'll also have an axis power. The axis number tells the eyeglass manufacturer where on your lens the astigmatism correction should be located.
ADD: The add number represents the magnifying power that will be added to the lower half of your progressive or bifocal lenses to help restore your near vision. (No more having to hold menus at arm's length.) This number is typically the same for both eyes.
Prism: The last two terms on the prescription are "Prism" and "Base." Some eyeglasses have a prism added because the person's eyes are not properly aligned. This is not a common problem, however, so oftentimes this field is left blank on your prescription.
The base indicates where the thickest edge of the prism will be: BU (for base up), BD (for base down), BI (for base in toward the nose), or BO (for base out toward the ears). The base direction tells you where the prism will redirect light.
Let’s put the above information into practice. In general, the further the number is from zero (whether positive or negative), the more correction is needed. Your optometrist or ophthalmologist will determine your visual accuracy and tailor your prescription accordingly to ensure your glasses provide the most clarity.
Here’s an example of something you might see on your prescription chart:
This means that the right eye (OD) has -2.00 of nearsightedness or 2 diopters. This is a relatively low amount of nearsightedness. The second column denotes -1.00 diopters of astigmatism, and this person will need a 100-degree axis to correct the issue. As you can see, the left eye (OS) is very similar, with slightly more nearsightedness and slightly less astigmatism.
These numbers will determine the thickness and curve of the lenses needed to correct the wearer’s vision and make it a perfect 20/20. If the nearsightedness was more severe, say -7.00 diopters, their glasses would need to be thicker. The same can be said for positive numbers, which indicate farsightedness. If the prescription reads +2.00, then the wearer needs 2 diopters of power to see things up close.
Since contact lenses sit directly on the eye, a special fitting is necessary in order to obtain the correct prescription for well-fitting contacts. Your standard glasses prescription is missing crucial information to ensure your contact lenses offer the same crystal clear vision that your glasses do. Luckily, your optometrist or ophthalmologist should be able to measure you for glasses and contacts at the same appointment. Your eyeglasses prescription will work for sunglasses without a problem.