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Behind the Scenes of our Vision: How Our Sight is Measured

The practice of an eye exam is a decades-old service that allows an eye doctor to see the big picture on your vision health. But, have you ever wondered how the routine sight test came to be? We take a closer look at the development of the modern day eye exam.


What is vision acuity?

This is also referred to as the sharpness of your eyesight. To measure your acuity, an eye doctor gauges your ability to identify letters on a standardized eye chart. This test is a static measurement, meaning you are not moving, and is performed under high-contrast conditions, which means the letters on the chart are black on a white background.


The Snellen Chart. Source: Wikipedia Commons


Your visual acuity is determined by how well the cornea and the lens of your eye focus on the retina, how sensitive the nerves in the retina and vision centres are in your brain, and how well you can interpret information that is received from your eyes.


This is an excellent way to determine how clear your vision is under standardized conditions; however, it doesn’t examine how well you can see objects in low contrast (similar brightness against its background), coloured objects, or moving objects.


Did you know that only light that is focused on a small, sensitive portion of your central retina (macula) influences visual acuity?


What does 20/20 vision mean?

The term 20/20 refers to fractions that measure your visual acuity. These fractions are officially named Snellen Fractions after the Dutch ophthalmologist Herman Snellen who developed the measurement system in 1862. He designed the Snellen Chart using specific targets in the font to measure visual acuity.


Despite the popular fonts in the era, he used letters that are equally thick, called optotypes. These optotypes are designed so that the thickness of the lines equals the thickness of the white spaces between lines and the height and width of each letter is five times the thickness of the line.


The numerator of the Snellen Fraction represents the 20 feet (six metres in countries that run off the metric system) a person is standing away from the eye chart.


Looking at the chart, if you can read the fourth line of letters from the bottom, you have “normal” vision acuity or 20/20 vision. The lines with smaller letters below the 20/20 line on the chart correspond to visual acuity measurements that are even better than 20/20 vision (e.g., 20/15, 20/12, 20/10).


The largest letter “E” at the top of an eye chart equals 20/200 visual acuity. If this is the smallest letter size you can see with corrective lenses, you are legally blind. This means that you can see the same letter that someone standing 200 feet away can see when you are standing only 20 feet away.


What is perfect vision?

Perfect or normal vision is considered 20/20 vision, but it is possible to see better than 20/20. It is common that people with healthy, young eyes can identify some letters on the 20/15 line or smaller.


Even though you may enjoy excellent vision, you may experience difficulties in other areas of sight. You can still be categorized as having normal vision in terms of viewing things from a distance, but could at the same time be hampered by other types of sight complications. For example, aberrations on your eye may affect your vision in the forms of problematic night vision, double vision, hallows, glare, or blurring. Although this may not affect your visual acuity, these aberrations reduce the sharpness of your sight.


If you have questions about how your eyesight is measured, make an appointment today at your Visique clinic.