Contact Lenses

Why Does My Eye Prescription Keep Changing?

Your Eye Prescription won’t stay the same throughout your lifetime. Here are the reasons why your vision keeps getting worse. Article by Joel Hunter, MD.

Joel Hunter, MD
Joel Hunter, MD
Refractive Surgeon, Hunter Vision Updated 11/24/20 2:04 PM

If you’re over 21 and your contacts or eye prescription changes rapidly or keeps changing, you might be surprised to know why. Well, it’s not what you think.


What do you mean by eye prescription?

If it’s your first time to visit your eye doctor, you may have most likely taken a basic eye exam (only to find out that you don’t have eagle eyes!). Chances are, you too were confused by the abbreviated terms such as OD, OS, OU, CYL and SPH. What do these mean?

When receiving your eyeglass prescription, you’d notice these abbreviations next to numerical values. And it’s not easy to comprehend what these letters, numbers and plus and minus signs mean. 

The first abbreviations you’ll notice are OD and OS. These refer to the Latin words oculus dextrus (OD), meaning the right eye; and oculus sinister (OS), meaning the left eye. When referring to both eyes, we use OU, short for oculus unitas (oculus uturque). Your doctor may not also use these abbreviations anymore as more optometrists and ophthalmologists now indicate LE (left eye) or RE (right eye).

What about the plus and minus signs? These refer to sphere (SPH or just S), which indicates the degree of your nearsightedness (–) or farsightedness (+).

On the other hand, CYL or just C, refers to the  cylinder or lens power needed for astigmatism measured in diopters. The higher the CYL, the more astigmatism there is.

So, if you have severe astigmatism, a sample prescription may indicate -5.25, which means you have 5 and ¼ diopters of nearsightedness. That can cause blurry and distorted eyesight. Don’t be surprised then why you’ll be asked to wear thicker lenses.

AXIS refers to the direction of correcting astigmatism measured between 0 and 180 degrees. Think of superimposing a protractor scale (yes, the one your trigonometry teacher forced you to use in ninth grade) on the center of the eye. The middle indicates 90 degrees, which corresponds to the vertical meridian, while 180 degrees corresponds to the horizontal equivalent. Knowing these angles helps your eye doctor determine the correct cylindrical power in your eyeglass lenses. 

There are also other terms you’d encounter such as DV (distance vision), NV (near vision), ADD (additional power for multifocal and bifocal lenses to fix presbyopia), and PRISM (amount of prismatic power used to correct eye alignment issues).

why eye prescription changes?

Are my eyeglass and contact lens prescriptions the same?

No, they’re not similar. While both prescriptions indicate farsightedness, nearsightedness and astigmatism (if it applies), there are other information on a contact lens prescription because the lenses are located on the surface of the eyes. On the other hand, your eyeglasses are about 10 to 12 millimeters (mm) from your eye surface.

So, what does 20/20 vision mean? Can I still achieve it?

A 20/20 vision means that you can interpret an image from 20 feet away. The image has to be noticeable from that minimum distance using the Snellen chart (the highly familiar eye chart with the big letter “E” on top).

The rule of thumb is this: the lower the second number, the clearer and sharper you’ll view objects at a distance. And yes, restoring your vision to 20/20 is usually possible. For an in-depth discussion of this, check out my article and video by clicking the link below:

What is “20/20” vision?

How often does eye prescription change?

If you got glasses for the first time before graduating high school, there is a good chance you’ve wondered why your prescription continues to fluctuate. 

A changing eye prescription is such a common phenomenon that it doesn’t even seem phenomenal. I’ll assume the correct word for this situation is a “nomenon” but there’s no time to look it up. We’ve got eye prescriptions to talk about.

The urgent matter of why your eye prescription changes is what most of this article will address. 

But the question of whether your eyes are getting worse is easier to answer. 

If you graduated high school before 2016 and were born after Gerald Ford was president, odds are the answer is “no.”

What factors cause my eye prescriptions to change so often?

It’s most likely that your eye prescription will have changed after you revisit your eye doctor. Aging is a major reason aside from illnesses that lead to eye complications. 

Your vision problems can get worse because of these factors:

Macular degeneration

Macular – what? Unknown to many, macular degeneration is the leading cause of vision loss according to the American Macular Degeneration Foundation (AMDF). The macula is a small spot in the center of the retina that helps us see objects straight ahead. When pigment changes occur in the retina, it may result in intermediate dry macular degeneration (which affects light-sensitive cells gradually) or wet macular degeneration (where abnormal blood vessels appear and leak blood).

Genetics, race and smoking can influence the occurrence of macular degeneration. While you can't control the first two, you can always decide to curb your cigarette cravings.


As you get older, the risk of having cataracts also increases. You'll experience blurry vision with cloud-like obstructions, faded colors of objects, too much brightness of headlights when you're driving at night, and even double vision. Doctors have identified that certain causes such as diabetes, extreme myopia, excessive smoking and drinking and even prolonged exposure to ultraviolet light can contribute to the development of cataracts.


Our optic nerve is made up of more than a million nerve fibers, which are connected to the brain. When it gets damaged due to the increased eye or blood pressure, one's vision is affected. Aside from blurred vision, patients may suffer from eye pain, nausea and reddening of the eyes.

Certain eye drops can treat glaucoma. Also, certain treatments such as Refractive Lens Exchange (RLE) can slightly lower the chances of contracting the disease.


If you have uncontrolled and erratic sugar levels that lead to diabetes, your vision will most likely suffer when left unrecognized. Diabetic retinopathy, or damage to the retina because of diabetes, may lead to vision loss when left uncontrolled and untreated. This occurs when weak blood vessels grow but leak and damage the retina.

So, go easy on the sugar before it’s too late.


This usually occurs when you’re in your early 40s. As you age, you lose your focus on small objects. For example, you find it hard to read your daughter’s mobile text message clearly even with your eyeglasses on. So, you adjust by holding your mobile phone farther from your eyes to lessen the blurriness.

Don’t panic though. Presbyopia is part of our aging process so visual fatigue will most likely come our way. But if you’re still not experiencing it, try to delay the symptoms by taking a break from playing 24/7 on your mobile phone.

Why does my vision get worse every year?

That prescription of yours keeps changing, though, doesn’t it? It seems impossible that a prescription can change while an eye isn’t changing shape. But it’s not only possible—it’s super common. 

We connect a changing prescription with a changing eye because, with the first few changes, it’s true. You are born with an eye of about 80% of the size it will be in adulthood. This means as your eye grows throughout your youth, the refractive error of your eye will change. That refractive error is what determines your actual prescription. Every millimeter the eye grows adds another -3.00 to its prescription.

Little babies who grow up to have a prescription of 0 have a prescription of +6.00 (!) when they’re staring up at the mobile rotating over their crib. There’s a lot of buffers here to allow the eye to grow without becoming nearsighted. Nearsightedness would have a minus prescription, meaning you have bad distance vision.

If you’re born with too little buffer (pretty much anything less than a +6.00 prescription), what happens? You get glasses at whatever young age your eye grows enough to become nearsighted. You hit 0 refractive error too early and move into a prescription starting with a minus sign. 

That refractive error will become larger as your eye gets larger. So a kid who gets his first pair of glasses in 4th grade (like I did) will need new glasses every couple years as his eyes grow. But at a certain age, eyes finish growing; that is 18 years old for 95% of people and 21 years old for everyone else. The same way your pants inseam doesn’t change in your twenties, neither does your eye.

How long is an eye prescription good for?

That prescription kept changing though, I bet. How? Well, we get used to having a changing prescription as we get taller, so when the prescription keeps changing after we’ve stopped growing it doesn’t strike us as odd. It becomes natural and common to need new glasses. 

So, you and the person who prescribes your glasses in your 20s and 30s never have a reason to question it. Neither of you stops to ask, “How on earth is this happening if my eye has stayed the same size and shape?”

We find this answer in the simple feature of acceptance. Accepting that refractive error (your eye’s needs) could change, means that a prescription (what an eye sees as optimal during an exam) often does change. In almost every case, your eye isn’t changing—the prescription is. 

There’s no mystery what allows this to happen. It’s the only other variable we can introduce at this point: the natural crystalline lens inside your eye.

Your lens was built to do one thing and do it well: focus. Without it, there would be no way to see objects any closer than about five feet away. We need a lens to autofocus and add extra power to see up close. 

It works perfectly in the real world. But that autofocus causes a lot of measurement error when you look through a phoropter. (That’s the device with a bunch of lenses that someone flips through and asks “better one…or two” in a hypnotic tone).

When those lenses are being flipped in front of your eye, your autofocusing lens— God bless it—is giving its all. It is trying to focus on every lens it can. All your lens can do is add focusing power (because that’s what you need for near vision, after all).

It comes out to play even more as those minus power lenses get higher and higher. If your eye stopped changing at a refractive error of, say -4.00, then -4.00 is all you need to see as well as you can.

What happens when your well-meaning eye doctor shows you a prescription of -4.50, though? It’s blurry for a fraction of a second before your natural crystalline lens says, “Wait, I got this!” 

That lens then adds its focusing power to bring the eye chart into clarity. That little bit of extra minus doesn’t add even a hint of blur after that. Guess what it does add, though: an indistinguishable amount of shrink to the eye chart. 

That shrink is enough for those letters to be more compact making them a little bolder. And that contrast looks nice. It looks nice enough that most people choose that lens. Instead of the lens that only made things perfect, people pick perfect plus bolder.

Now you wear those overpowered (technically called overminused) glasses for a couple of years. Then, the next time you pick lenses, your starting point for “normal” is that little bit bolder version. 

The next lens up from there will be blurry for a fraction of a second. But when it snaps into focus… whoa! Now you’ve got an eye chart that is even bolder and higher contrast. And you’ve got a prescription that’s now two generations stronger than the one you actually need.

This is so common, it’s expected. We expect it when consulting and testing for LASIK. It’s also considered when deciphering the true refractive error of an eye versus the prescription in the current pair of glasses or contacts. I once had a patient in -6.00 glasses whose actual correct prescription was half that at -3.00! Her eyes felt a lot less tired all the time after LASIK.

For most people, the difference isn’t as shocking. It is enough, though, that they’ve watched their prescription change over the years. It leaves many people wondering when their prescriptions will stop changing. In almost all cases, the answer is their eyes stopped changing years ago. 

Their prescription would have stopped changing then too if it was measured correctly. To be fair, the wonderfully accommodating lens does make it hard to measure prescriptions with the accuracy of an inseam. There’s a reason it’s such a common nomenon.

Seeking Affordable Treatment

If you experience eye prescription changes every now and then, seeking the help of an eye doctor should be your priority for a thorough examination. Your doctor may also help you detect other eye issues that can be prevented early.

But are you ready for the cost?

At Hunter Vision, we fully understand the value of taking care of your eyes without the expensive treatment procedures. You get expert treatment from one of Florida's best eye doctors BUT without asking you to dig a hole in your pocket.

Our affordable payment plans for vision correction surgeries including LASIK can fit your tight budget. That's why we offer payment plans for as low as $130 per month - yes, that's not a typo - so you can start and end your day without your eyeglasses.

Contact Hunter Vision today to schedule your appointment or call 407-385-1620.


Author:Joel Hunter, MD is an Ophthalmologist, Refractive Surgeon, and the Founder of Hunter Vision, a LASIK Clinic in Orlando, Florida. A recognized and respected specialist in vision correction who has performed a countless number of refractive surgeries, Joel gives lectures across the country and trains fellow doctors in the newest LASIK surgery techniques.

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