What 20/20 Vision Actually Means

One of the most commonly asked questions in an eye exam comes right after the refraction, or glasses prescription check: “What is my vision?”

Almost invariably, people know the term “20/20”. In fact, it’s a measure of pride for many people. “My doctor says I have 20/20 vision.” Or, on the other side of that same coin, having vision that is less than 20/20, say 20/400, can be a cause of great concern and anxiety. In this discussion I will describe what these terms actually mean.

To lay the foundation, let’s discuss some common terms. Visual acuity (VA) is clarity or sharpness of vision. Vision can be measured both corrected (with glasses or contact lenses) and uncorrected (without glasses or contact lenses) during the course of an eye exam. The result of an eye exam boils down to two different but related sets of numbers: your VA and your actual glasses prescription.

The notation that doctors use to measure VA is based off of a 20-foot distance. This is where the first 20 in 20/20 comes from. In Europe, since they use the metric system, it is based on meters. The 20/20 equivalent is 6/6 because they use a 6-meter test distance. The second number is the smallest line of letters that a patient can read. In other words, 20/20 vision means that at a 20-foot test distance, the person can read the 20/20 line of letters.

The technical definition of 20/20 is full of scientific jargon - concepts such as minutes of arc, subtended angles, and optotype size. If you’d like to read more of the technical details there is a well-written article with illustrations by Dr. John Ellman, you can find here.  For the purposes of our discussion here I’ll try to explain it in less technical terms.

“Normal” vision is somewhat arbitrarily set as 20/20 (some people can see better than that). Let’s say you have two people: Person A with 20/20 vision and Person B with 20/40 vision. The smallest line of letters that person B can see at 20 feet is the 20/40 line.  Person A, with “normal” 20/20 vision, could stand 40 feet away from that same line and see it. There is somewhat of a linear relationship in that the 20/40 letters are twice the size of the 20/20 letters and someone with normal vision could see a 20/40 letter at twice the distance as the person with 20/40 vision.

So how does this translate to a glasses prescription?

Eye doctors can often estimate what your uncorrected VA will be based on your glasses prescription. This works mainly for near-sightedness. Essentially, every quarter step of increasing glasses prescription (i.e. -1.25 as compared to -1.50) means a person can see one less line on a VA chart.

A prescription of - 1.25 works out to roughly 20/50 vision, -1.50 to 20/60 and so on. Anybody with an anatomically sound eyeball, meaning the absence of any kind of disease process, should generally be correctable to 20/20 with glasses or contact lenses. It is important to note, however, that rarely a person’s best corrected VA may be less than 20/20 with no noticeable signs of disease.

Far-sightedness is more difficult to estimate because it is affected by a number of other factors, including one’s age and focusing ability. But that’s a topic for another article.

So there you have it! Hopefully this has shed some light on what these measurements that we take actually mean, and it has allowed you to understand your eye health a little bit better.

Should I Be Scared of My Astigmatism Diagnosis?

The word “astigmatism” is used so much in the optometric world that most people have talked about it when discussing their eye health with their doctor.

“Astigmatism” comes from the Greek “a” - meaning “without” - and “stigma” - meaning “a point.” In technical ocular terms, astigmatism means that instead of there being one point of focus in the eye, there are two. In other words, light merges not on to a singular point, but on two different points.

This is experienced in the real world by blurred, hazy vision, and can sometimes lead to eye strain or headaches if not corrected with either glasses or contact lenses.

Astigmatism is not a disease. In fact, more than 90% of people have some degree of astigmatism.

Astigmatism occurs when the cornea, the clear front surface of the eye like a watch crystal, is not perfectly round. The real-world example we often use to explain astigmatism is the difference between a basketball and a football.

If you cut a basketball in half you get a nice round half of a sphere. That is the shape of a cornea without astigmatism.

If you cut a football in half lengthwise you are left with a curved surface that is not perfectly round. It has a steeper curvature on one side and a flatter curve on the other side. This is an exaggerated example of what a cornea with astigmatism looks like.

The degree of astigmatism and the angle at which it occurs is very different from one person to the next. Therefore, two eyeglass prescriptions are rarely the same because there are an infinite number of shapes the eye can take.

Most astigmatism is “regular astigmatism,” where the two different curvatures to the eye lie 90 degrees apart from one another. Some eye diseases or surgeries of the eye can induce “irregular astigmatism,” where the curvatures are in several different places on the eye’s surface, and often the curvatures are vastly different, leading to a high amount of astigmatism.

Regular astigmatism is treated with glasses, contact lenses, or refractive surgery (PRK or LASIK). Irregular astigmatism, such as that caused by the eye disease keratoconus, usually cannot be treated with these conventional methods. In these circumstances, special contact lenses are needed to treat the condition.

The next time you hear that either you or a loved one has astigmatism, fear not.

It is easily corrected, and although astigmatism can cause your vision to be blurry, it rarely causes any permanent damage to the health of your eyes.

If you experience blurred vision, headaches, or eye strain, having a complete eye exam may lead to a diagnosis and treatment of this easily-dealt-with condition.

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