How bad is my eye prescription?
Many of my patients ask me, “How bad is my eye prescription?” or “What eye prescription is considered bad?” Sometimes they’ll ask “What eye prescription is legally blind?” or “What is a legally blind prescription?”
These are great questions. Most everyone who wears glasses or contacts thinks their eyes are extremely nearsighted, when in fact most nearsighted people are just mildly or moderately nearsighted. However, when it comes to functioning in the world it doesn’t take much nearsightedness to seriously decrease your ability to drive, watch TV, or do just about anything.
The difference between mild and severe nearsightedness is just a few inches. Those with mild nearsightedness (-2 prescription) start to have blurry vision for objects more than 20 inches away, and those who are very highly nearsighted (-8 prescription) start to see blur for objects more than 5 inches away. so it’s 5 inches as compared with 20 inches. Not a lot. Blurry versus very blurry doesn’t make much difference when it comes to driving or seeing someone or something from across the room or across the street. If you can’t see, you have to do something about it.
Making glasses a thing of the past is the mission of SharpeVision. It’s easier than you can imagine. Come to your free comprehensive exam to find out. https://sharpe-vision.com
Understanding your Prescription.
I’ll try to make this explanation of your glasses prescription as user-friendly as I can. It’s not always easy to understand but it is fairly simple overall. First I will describe the components of your glasses prescription, then describe what ranges are average. I will talk a bit about glasses prescriptions worldwide. I will address the question of “how strong is my eye prescription?” bad eyesight numbers, negative vision scale, and negative eyesight. I will finish up with a discussion of the definition of “legal blindness“ It’s important to note that the vision chart and eye prescription chart (glasses prescription) while they are related, do not necessarily correlate exactly. I will talk more about this later.
Your glasses prescription is fairly simple and consists of three parts:
- Sphere: Nearsightedness/farsightedness
- Cylinder: Astigmatism
- Axis: Direction of the astigmatism between 1° and 180°
The abbreviations are as follows:
- OD, oculus dexter, or right eye
- OS, oculus sinister, or left eye
- OU, oculus uterque, or both eyes
- “Add” is adding focusing power, typically +1.25 to +1.75, which is the added focusing power required in someone over 45 years old to help them clearly see near objects.
- PD, or pupillary distance. This is the distance between the center of your pupils which is important for making the glasses so that the center of the lens is over the center of your pupil.
Sphere: this tells the person making your glasses if your eyes need more or less focusing power to see clearly. It makes sense that if your eyes don’t have enough focusing power, the symbol is a plus sign “+” If your eyes have too much focusing power, the symbol is a minus sign “-“ A plus sign is farsighted, and a minus sign is nearsighted. So nearsighted people’s eyes focus too much, and distant objects are blurry. Nearsighted people wear glasses that are concave, or thinner in the middle and thicker on the edges. This makes the light rays diverge, or focus less, so they’re called minus lenses and take away focusing power. Farsighted people wear glasses that are thicker in the middle and thinner on the periphery. This makes light rays converge or focus more. They are commonly known as magnifying lenses and are given a “plus” power that adds focusing power to allow farsighted people to see clearly.
A farsighted person’s eyes don’t have enough focusing power, so they have to use muscles to squeeze the lens of their eyes to add focusing power. If they can’t squeeze the lens enough, their vision is blurry.
One additional helpful fact is that to see objects up close, our eyes need more focusing power. That explains why nearsighted people can see well up close. Their eyes have too much focusing power to see distant objects, but that helps up close. The more nearsighted they are, the closer they can see clearly.
A second very important fact is that our eyes have a flexible lens that can add focusing power. When we’re under 45 or so years old, that helps us to add focusing power for reading or any near object. As we age and the lens gets less flexible and can add less focusing power, we begin to need what is called “add” or bifocals to see near objects.
Astigmatism is the part that’s the most confusing, so I’ll discuss that briefly, and I hope you’ll understand better than you might have in the past. To understand astigmatism, imagine a tennis ball. Now picture it cut it into two equal halves. If you were to take this half of a tennis ball and hold it at the cut edges with your thumb and index finger and squeeze slightly, you’ve created the shape of your eye with astigmatism. The more you squeeze, the more astigmatism you create. This can be measured and is put into the glasses prescription. It indicates that your eyes focus differently in different parts of the front of your eye. For example, if you focus perfectly in the horizontal (left to right, or straight across) position, but have one unit (called a diopter) too much focusing in the vertical (up and down) position, then your prescription would read: plano (zero) -1.00 at 180°
Note: plano means zero, and don’t worry at this point about 90° (up and down) as compared to 180° (straight across the horizon). That’s all I’ll discuss astigmatism here, but almost everyone has some degree of astigmatism. It just means our eyes aren’t perfectly round.
How bad is my eye prescription? What is considered bad eyesight numbers? Most people with glasses are nearsighted, which is why there is more talk about “negative vision scale” and negative vision numbers.
Nearsightedness is categorized into mild, moderate, high, and extreme:
- Mild: -0.50 to -3.
- Moderate: -3.25 to -5.00
- High: -5.25 to -10
- Extreme: greater than-10
WHO and CDC: Glasses prescriptions worldwide.
A recent (2015) World Health Organization (WHO) described an alarming trend to increasing nearsightedness throughout the world. https://www.who.int/blindness/causes/MyopiaReportforWeb.pdf#12
This report shows how nearsightedness is increasing, and how and why this can be detrimental to both the health of individuals and populations. Legal blindness can be caused by nearsightedness, but generally is misunderstood by my patients. I’ll explain it next.
Legal blindness is defined more by your government than by your eye doctor, but your eye doctor can help get you declared legally blind so that you can get help. Those who cannot see well enough even with glasses or contacts to operate a motor vehicle or have difficulty performing the duties of a regular job obviously may have difficulty earning a living, and therefore may qualify for government aid, low vision devices, vocational training, and other government help. This is the basis for the “legal blindness definition.
Legal blindness is defined as: 20/200 or worse vision in the better seeing eye, or less than 20° of a field of vision. For instance, I remember a patient I saw as a resident in training who had 20/15 (better than 20/20) vision with no glasses on, but who had lost visual field from glaucoma to the point where his vision was like looking through a paper towel tube. He could only see a very narrow range, and had virtually zero peripheral vision. The most common causes of legal blindness are due to macular degeneration, diabetic retinopathy, glaucoma, and trauma. Cataract in the developed world has largely been eliminated due to excellent eye surgery that can replace the cloudy aging lens of the eye with a clear artificial lens.
People with glasses or contacts that correct a person’s vision so they can see well therefore do not meet the definition of “legal blindness” although it sounds bad!
The concerning part of increasing prevalence and severity of nearsightedness throughout the world is that severe nearsightedness can lead to conditions that glasses cannot fix! Pathologic myopia is a condition where the retina is damaged by the increase stretching and lengthening of the eye. Glaucoma, cataract, retinal detachment are all potentially blinding conditions that are increased in highly nearsighted people.
This is why bad eyesight numbers, negative vision scale, and negative eyesight, are all worrisome to those in the WHO, CDC, and national health organizations whose mission it is to define, monitor, study, and help find solutions to health problems such as worsening nearsightedness in the world. I’m glad we have these people watching over us and doing their best to find solutions.
-Dr. Matthew Sharpe