Have you ever heard that 20/20 vision is when you can see something at 20 feet that someone with normal vision sees at 20 feet? What if I tell you this is actually not true.

The True Meaning of 20/20 Vision

If this definition is wrong, then why is it such a commonly accepted answer? Just search the meaning of 20/20 vision and you'll be flooded with this answer. Many from sources you would consider reliable.

This misinformation has become so common because it is much easier to explain and understand than the actual truth - math.

The Math Behind 20/20 Vision

When standardizing a measurement system for vision, early opticians relied on mathematical optics to decide how far a patient should stand and what the size of the letters should be. They determined that a letter that subtends 1 arc minute on the retina of a person standing 6 meters away should be 8.75mm high.

Because of this mathematics, the true definition of 20/20 vision is the ability to see a letter that subtends 1 arc minute while standing 6 meters (20 feet) away.

What Is an Arc minute?

An arc minute is a unit of angular measurement. The angle 1 arc minute is defined as 1/60 of one degree or 1/21,600th of a complete circle.

In turn, 1/60 of an arc minute is 1 arc second.

1 arc minute is very important in the early days of optics as scientists at the time determined that this is the smallest angle at which a person can distinguish two points. This means that the human eye can perceive two points as separate as long as the distance between them is at least 1 arc minute.

How To Measure Vision

Your Doctor uses equipment that is standardized and calibrated to very specific measurements in order to accurately assess your vision.

While there are many different versions and shapes in the eye chart, the one constant is that the 20/20 line of letters will subtend no more and no less than 1 minute of arc in your eye.