What 20/20 Vision Really Means

Eugenia Cheng is writes a column for the Wall Street Journal Weekend Edition named “Everyday Math”, and her piece this weekend is titled What 20/20 Vision Really Means. She writes: “A friend of mind recently had laser eye surgery, and after which he was declared to have ’20/15 vision.’ I was surprised by this way of describing vision that is even better than 20/20 and realized that, despite wearing glasses since the age of 4, I’ve never though about what 20/20, or indeed my eyeglass prescription, means.”

You might be tempted to correct Dr. Cheng’s use of the term “vision” in conjunction with the Snellen numerator and denominator, but the fact that she is a bright and accomplished individual, a concert pianist with a Ph.D. in mathematics from the University of Cambridge, may be a sign that hammering away at the distinction between “vision” and “eyesight” all these years is largely a losing battle.

Dr. Cheng continues: “It turns out that 20/15 is not describing left and right eyes, nor does it represent a score of higher than 100%. Informally, 20/15 means that at a distance of 20 feet you can see what an average person can see at a distance of 15 feet. This means that you can see somewhat better than average. Thus 20/20 isn’t a description of perfect clarity of vision but of average vision for humans.”

I suspect that you and I would agree this is a bit confusing. Nowhere does Dr. Cheng state that the de facto definition of 20/20 is built around seeing a certain size letter at a specified distance. In my mind, at least, that would have set the stage better for what comes next. She writes:

“What matters to our vision is nether the size of the object nor the distance by themselves. Rather, it’s the angle that the edges of an object make on our retina, which takes into account the trade-off between the size of the object and how far away it is …

The question for measuring our visual acuity, then, is this: What is the minimum angle our eye can detect? That answer is called the Minimum Angle of Resolution, or MAR, measured in arcminutes, which are each 1/60th of a degree. A little trigonometry translates one arc-minute to a height of around 1/16 of an inch at a distance of 20 feet, but the fraction of an inch isn’t what matters. This level of acuity – one arcminute – has been designated as the baseline 20/20, or average.”

The readership of the Wall Street Journal, in which this column appeared today, is brighter than average. But there is reason to doubt that few of them understands what 20/20 vision is – or at least any better than they did prior to reading the column. This is through no particular fault of Dr. Cheng because it is, in fact, a complex topic to explain. I’d respectfully suggest that had she used a graphic aid, perhaps something like this, it might have helped the explanation:

As Optometry students learn early on, the concept of 1 minute of arc designating a “20” size letter at a viewing distance of 20 feet corresponds only to the angle subtended by one stroke of the E (or the width of the gap in a Landolt C). The overall size of the letter actually subtends 5 minutes of arc. This is essential in calibrating the Snellen eye chart based on viewing distance. Furthermore, Optometry students learn the distinction between minimum resolvable and minimum recognizable acuity, which influences what we mean when we describe how sharply someone can see at a particular distance.

The reason I said earlier that you can’t fault Dr. Cheng for conflating vision and acuity is that even the American Optometric Association does it. Have a look here:

Right after the statement above, the AOA page elaborates: “Having 20/20 vision does not necessarily mean you have perfect vision. 20/20 vision only indicates the sharpness or clarity of vision at a distance. Other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and color vision, contribute to your overall visual ability.” That is helpful in getting the point across that vision is comprised of many factors beyond acuity. But it would be less confusing to the public if we said that 20/20 is a measure of eyesight, and make no mention of vision in connection to that troublesome fraction.