As an optician, Alistair's primary expertise is in vision health. He has also been researching nutrition science for eleven years.
There are an infinite number of iterations and combinations of eyeglass prescriptions prescribed by opticians, opthamologists, and retina experts. This article will help explain what each of the numbers on your prescription mean and why you don't need a trained professional to interpret them for you.
How to Read an Eyeglass Prescription
- Step 1: Learn the sphere number
- Step 2: Learn the cylinder and axis numbers
- Step 3: Understand how the sphere, cylinder, and axis numbers work together
- Step 4: Understand how to read bifocal additions
Step 1: Learn the Sphere Number
First, before you see examples of eyeglass prescriptions, you'll need to familiarize yourself with a few terms:
- OD is an abbreviation for oculus dexter, Latin for right eye (from the patient's point of view).
- OS is an abbreviation for oculus sinister, Latin for left eye (from the patient's point of view).
- Nearsightedness, or myopia, is when someone is unable to see things clearly unless they are relatively close to their eyes. This is caused by the focusing of rays of light by the eye at a point in front of the retina. This is the most common refractive error of the eye, and it has become more prevalent in recent years. In fact, a study by the National Eye Institute (NEI) shows the prevalence of myopia is growing rapidly. Although the exact cause for this increase is unknown, many eye doctors feel it has something to do with eye fatigue from computer use and other extended near vision tasks.
- Farsightedness, or hyperopia, is when someone is unable to see things that are relatively close to their eyes. This is caused by the focusing of rays of light by the eye at a point behind the retina. While this is less common than nearsightedness, farsightedness is still a major problem for many people around the world.
- Having plusses and minuses for your sphere number (ignoring astigmatism, macular degeneration, or the need for a bifocal) represents your ability to see objects far away, and in some cases, up close. Minus prescriptions mean you are nearsighted and are able to see objects close up (and for some of the highest prescriptions, such as -15.00, the person can only see what is right in front of their face), and plus prescriptions mean that you have trouble seeing what's in the distance (beyond ten feet).
Lens Prescription for Nearsightedness
OD (Right Eye)
OS (Left Eye)
In this case, the sphere is -1.25 for the right eye, and -1.50 for the left eye. There is an important point to be made. Regardless of whether or not there is a + or - before the sphere number, the rule of thumb is: The higher the number, the worse the eyesight. So a -6.50 is always worse than a -2.50, and a +7.50 is always worse than a +2.50.
So, in the situation above, the left eye is worse than the right. It's a marginal difference, but if we were to assume -1.25 for each eye, the left eye would be straining slightly to see as clearly as the right. In turn, that could cause further issues, such as headache, fatigue, and migraines.
Further Explanation for Plus and Minus Prescriptions
Say a person with a minus prescription wanted to read a book, they would tend to have it right in front of their eyes. A person with a plus prescription would try to read the book at arm's length (which, due to other complications, is not always possible).
Now you know what the sphere property of the prescription is. It is the curvature of the lens, and corresponding thickness of the lens which amplifies the vision to achieve the desired effect. A -10.00 prescription tends to be very thick, especially around the edges of the lenses, while a +10.00 prescription will be very thick and bubbled in the middle of the lens. It's all about correcting the issue.
Step 2: Learn the Cylinder and Axis Numbers
The cylinder is a measure of one's astigmatism. Astigmatism is an abnormality in the shape of the eye that causes light entering the retina to become distorted and sent in the wrong direction. People with heavy astigmatism mimic those with heavy plus sphere prescriptions. They cannot see far away or close up. It's a level of haziness and blurriness that will cause a person to focus perpetually on something that they will not be able to see clearly. A person with heavy astigmatism will have eyes that look like footballs rather than soccer balls. Luckily, this is something that can be corrected with eyeglasses.
The axis describes the lens meridian that contains no cylinder power to correct astigmatism. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.
Read More From Healthproadvice
In the previous example, there was no number listed for astigmatism. This means that the patient does not require any correction in that area. Let's use a new example.
Lens Prescription for Farsightedness
OD (Right Eye)
OS (Left Eye)
In this example, both eyes are farsighted, and have heavy astigmatism. It is extremely rare to find a patient with -4.00 astigmatism. This patient would be essentially blind as a bat without glasses. The cylinder twists the sphere about an axis, causing the lens shape to change in nature, and curvature. Your cylinder and axis numbers are determined during your optical exam.
Step 3: Understand How the Sphere, Cylinder, and Axis Numbers Work Together
The lenses in your glasses compensate for your eyes' natural defect by changing the direction in which the light comes through the lens, and at what angle and magnitude. People with high sphere, cylinder, and axis numbers rely on glasses and contact lenses more than someone with low numbers. For example, someone with a +5.00 sphere would need glasses more than someone with a +1.25 sphere.
The cylinder is displayed using a negative number (the higher the negative number, the worse the astigmatism), unless you are seen by an opthamologist. They have the option of writing it as a positive number, but there's a mathematical formula that opticians follow to convert their positive annotations to the correct negative ones.
The axis is always on the range from 0 to 180, where 0 and 180 are basically equivalent. It indicates how much the lens must be adjusted about the axis to correct which way the light enters the eye. Low numbers and high numbers are essentially equivalent. There is no "bad" axis number, only different ones.
The combination of these three elements explains how curved and thick your lenses will need to be. The size, shape, and thickness of your glasses relate directly to the sphere, cylinder, and axis numbers. Think about how on tv shows the elderly are often displayed wearing glasses with thick lenses. The worse your eyesight is, the more your glasses will be shaped to compensate for the impairment.
Step 4: Understanding Bifocal Additions
Bifocal prescriptions are written as a +x.xx after each of the other numbers, and most doctors either specify PAL or FT for their prescriptions, which mean no-line or line bifocals, respectively.
In the scenario below, the patient is nearsighted, with -3.00 and -3.75 spheres. They also have some hindrance from their +1.25 and +1.75 cylindrical.
In this case, the bifocal wearer needs a strong bifocal of +2.50. Essentially, this works just like a magnifying glass. It amplifies things up-close, but is useless at a distance. The doctor specified PAL which means they need a progressive bifocal.
When it comes to progressive bifocals, there is a small area at the very bottom of your lens where you can utilize that +2.50 magnification. Don't confuse your bifocal +2.50 number and your +2.50 sphere number. They're completely unrelated. A +2.50 sphere with a +2.50 reading addition, does not equal +5.00 magnification.
The bifocals are meant for objects within five feet, but usually more around the three-feet-and-closer range. It magnifies whatever you're looking at, and makes up for your eye's natural tendency to become unable to focus on objects up-close.
Be thankful if you only have a +1.50 or +1.75 reading addition, as that means you could technically get by without glasses for reading. However, it would be difficult. You'd end up having to put the book at arm's length and squint.
Many people who have low spheres and no astigmatism will still have some problems when reading up-close. They usually choose to buy reading glasses, which serve the same exact purpose as bifocals, but don't correct for any of your other vision problems.
Mild and Intense Lens Prescriptions
Right (mild nearsightedness)
Left (mild nearsightedness)
Right (intense nearsightedness)
Left (intense nearsightedness)
Right (mild farsightedness)
Left (mild farsightedness)
Right (intense farsightedness)
Left (intense farsightedness)
Prisms and Universal Eyeglass Prescriptions
Prisms: Prisms are a rare occurrence in the optical world because, simply put, not many people need this specific correction. Prisms are specially built in-lens adjustments that compensate for a specific eye issue. This eye issue occurs if one of your eyes, or both, are not in line with each other (meaning that they cannot focus on the same object). For lazy-eyed folks, prisms are useless. However, for people who, due to a birth defect, have an eye looking slightly off in a different direction, prisms correct the problem of double vision.
Eyeglass Prescriptions are Universal: Sometimes they will omit this fact at doctors offices so that you stay there and buy what they have to offer. Don't forget that you can take your prescriptions (and you are entitled to your prescription) anywhere and everywhere.
Closing Thoughts on Eyeglass Prescriptions
Hopefully this article has helped explain a bit about what an eyeglass prescription is, and how to read/decipher it. I'm sure, by now, you're calculating just how bad your eyes really are. I'm just happy and glad that modern science has gotten optometry and eye-care down to such a solid routine. Sure, yearly exams are usually required, and sure, eyeglasses fall apart pretty easily, but there are very few secrets in the field of optometry. As a consumer and patient, you have a right to know how to read a prescription so that you can make informed decisions. I hope this article has been helpful and I wish you all the best of luck in your eye-care endeavors!
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.
Dee L on October 16, 2018:
Very Helpful and informative!
Sophie on November 08, 2017:
I really liked this article, very easy to follow and it answered everything I was looking for. Thank you :)
Dr M Ashraful Kabir on August 30, 2017:
here not clear D.V., N.V., PAL/FT
Tren on March 05, 2017:
Great article - best at explaining in easy and understandable terminology.
B. Hyman on February 21, 2017:
Author... what are your qualifications for writing such a horrendous, difficult-to-follow, and inaccurate article?
Bunda on January 14, 2015:
its from their genetics. most of the time you have one of your two paenrts color eyes but sometimes you have both. it is kind of weird. also sometimes crack babies have different colored eyes but still they can't help it