Melissa Flagg, COA, OSC, has worked in the medical field for over two decades as an ophthalmic technician and is certified by JCAHPO.
Trying to read any prescription written by a doctor is like trying to decipher Egyptian hieroglyphs, but a glasses prescription can be even more difficult. They look almost like algebraic equations. But these prescriptions are actually much easier to understand than those written for medications.
Refractive error refers to the refractive index of the eye, specifically the amount of “error” that is causing the patient’s blurry vision. This is what glasses correct.
There are three different types of refractive error:
- Myopia (a person who is nearsighted)
- Hyperopia (a person who is farsighted)
- Astigmatism (someone whose cornea is irregular)
Refractive error is measured in diopters, which can be either negative or positive. The power of a person’s lens can be changed in quarter diopter steps. If we think of diopters in terms of money, this is easier to understand. For example:
One diopter is equal to $1. A quarter of a diopter is equal to $0.25. Half a diopter would be $0.50, and so forth.
Probably the most common form of refractive error, myopia is the inability to see clearly at distance, but vision at close range is perfect. This is otherwise known as “nearsightedness.”
Myopia is caused by an eye that is either too long, a lens that has too much magnification (or plus power), or a combination of the two. In each case, the image that is seen falls in front of the retina, causing the image to be blurry.
To correct myopia, convex lenses are used. These lenses are measured in negative diopters, typically called "minus power" in ophthalmology. If held over text on a piece of paper, a convex lens will make the letters look smaller.
The second most common form of refractive error is hyperopia, or farsightedness. These patients can usually see fine at distance, but have difficulty seeing clearly when reading or doing any type of close work. Some patients with extreme hyperopia have difficulty seeing things clearly at any distance.
Hyperopia is the result of an eye that is too short, a lens that does not have enough plus power, or a combination of these two. This causes the image to fall behind the retina, causing a blurry image.
To correct hyperopia, concave lenses are used, which are measured in positive diopters or "plus power." If placed over text on a piece of paper, concave lenses will magnify the letters, which is why these lenses are used in over-the-counter reading glasses.
Astigmatism is the term given to a cornea that is irregular. The cornea is the shape of a dome and is, essentially, the eye’s window—providing 70 percent of the eye’s focusing power.
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Think of this dome as a freshly paved road. It should be smooth and free of bumps and other irregularities. With astigmatism, however, this road has potholes and bumps. In some cases, this road may even have speed bumps.
Astigmatism is corrected with a spherocylindrical lens, more commonly known as cylindrical or cylinder lenses.
Common Ocular Abbreviations
Oculus Dexter (Right Eye)
Oculus Sinister (Left Eye)
Oculus Uterque (Both Eyes)
Literally Addition meaning the power of the bifocal lens
Since there are three main different types of refractive error, there are three distinct sets of numbers in a glasses prescription, and they are written in a specific order:
- Sphere (written as sph)
- Cylinder (written as cyl)
- Axis (typically a three digit number such as 005 or 165)
Each of these numbers corresponds to the correction required by the eye to focus the image directly onto the retina.
When your doctor performs a refraction using a phoropter (like the one in the picture above) to determine your prescription, he or she checks each one of these individually and in the order written above.
The sphere indicates whether a patient is nearsighted or farsighted, and you can determine this by looking at the sign in front of the numbers.
If it’s positive, the patient is farsighted; if it is negative, the patient is nearsighted. The vision gets progressively worse as the number of the sphere gets larger. For example, someone who is a -1.00 sphere has better vision than someone with a -5.00 sphere.
Cylinder and Axis
The cylinder indicates whether or not a patient has astigmatism, and as with sphere, the higher the number, the blurrier the vision. But this is where a prescription can be a bit tricky.
Cylinder is always associated with an axis, which tells you where on the cornea the astigmatism is located. The axis is a number between 1° and 180°.
The cylinder can be either negative or positive.
- If the prescription was written by an ophthalmologist, the cylinder will be positive, which is called “plus cylinder.”
- If an optometrist wrote it, the cylinder will be negative, which is termed “minus cylinder.”
With a bit of math, you can easily transpose one cylinder to the other.
This is my glasses prescription in both plus and minus cylinder:
Typical Spectacle Prescription in Plus Cylinder
The Same Prescription in Minus Cylinder
The "add" power refers to the strength of the bifocal. It’s called an add because it is plus power that is added to the sphere of the prescription.
So, in my prescription above, if we add +1.00 to the sphere of the right eye, which is -5.25 (in the minus cylinder example), my bifocal power will actually be one diopter less than my distance power, or -4.25. If the sphere number was positive, and the add power was +1.00, the bifocal power would be one diopter more than the distance power.
Prism and Base
There are two other boxes on the prescription that we have not discussed: Prism and Base. In most cases, these boxes are empty. People who suffer with double vision and extraocular muscle imbalances are typically prescribed prism.
Prism is measured in "prism diopters" and have a base direction. A prism in its simplest form is a triangle that focuses light in a certain direction. The direction of the base of the prism will tell you which direction the light will move.
For those with strabismus (muscle imbalances) or diplopia, a prism allows them to see one image again by altering where their eyes look. For example, if a patient has an eye that turns outward (toward the temple), adding a base out prism (meaning the base of the triangle faces out toward the temple area) will cause the deviating eye to turn inward, correcting the displacement.
Again, most people will never see prism or base written on their prescription. However, if you do, it will most likely be written as 4ΔBO (for a 4 diopter prism with the base facing out). Prism diopters can have four different bases:
- Base Out (written BO)
- Base In (written BI)
- Base Down (written BD)
- Base Up (written BU)
Each base direction corresponds to a deviation of the eye, whether the eye is turning inward, outward, up or down.
There you have the basics of reading an eyeglass prescription.
A Note About Contact Lens Prescriptions
A contact lens prescription differs slightly from an eyeglass prescription, since there is additional information that describes the lens size, base curve, etc. However, the power of the lens is written in the same manner.
This content is for informational purposes only and does not substitute for formal and individualized diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed medical professional. Do not stop or alter your current course of treatment. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
© 2013 Mel Flagg COA OSC
Kristen Howe from Northeast Ohio on February 10, 2016:
Melissa, this is a handy hub about eyeglass prescriptions. I'm going to be needing reading eye glasses this spring to help out with my eye strain. I used to wear glasses when I was younger for a few years. This is good to know.
Mel Flagg COA OSC (author) from Rural Central Florida on July 21, 2013:
@Frank, thank you!! I'm surprised I waited to long to write this one, but so many people ask me what all the numbers mean during exams that it seemed like a much needed hub! :)
Frank Atanacio from Shelton on July 14, 2013:
Daughter of Maat, I never even thought of ever reading a hub on eyeglass prescriptions.. but actually this was useful.. and important thank you so much voted way up :)
Zulma Burgos-Dudgeon from United Kingdom on July 01, 2013:
You certainly know your stuff. It was an interesting read but I don't think I'll try to interpret my script. I'll leave that to the professionals. lol