How about the next line? We have all anxiously squinted to see the blurry lines during an eye test-like we are getting a grade or something. We have a sense of satisfaction when we call off all the letters, and then are somewhat embarrassed when we realize that the F was a P and the B was an F.
I’ve worn glasses since the 5th grade, and the method of testing my visual acuity has changed very little. Herman Snellen’s eye chart was a big hit in Europe, and then, in 1861 the British Army decided to order the charts to test their soldiers vision. They wanted their soldiers to be the best at firing their muskets, and that required good vision. This low tech solution was the first to address the complex issue of visual acuity.
If you had poor vision in the 1700’s you really had a problem. Glasses were sold out of baskets by street vendors. Now I spend an hour or two trying on different frames that will hold very expensive lenses that have been ground to a delicate solution for my less than 20/20 vision. It is the Snellen Chart that helps an optometrist calculate your vision using the 20/20 model.
The idea is that 20/20 is the normal visual acuity of the average person-meaning that standing 20 feet away from something, you can see what the average person can see standing 20 feet from he same thing. Since most doctor’s offices are too small for 20 feet, mirrors will be used to simulate 20 feet distance from the eye chart. Most people can read the fourth line up from the bottom without any trouble, meaning that your vision is 20/20 if you can read that line. Alas, my vision is 20/450 and 20/650 making me blind except for the miracle of corrective lenses which bring my vision to 20/30. Just as a reminder of what those numbers (20/450 and 20/650) mean: I can see at 20 feet what most people can see clearly at 450 feet or 650 feet.
What is your vision? There is a really good chance you need corrective lenses-in the United States, it is believed that 75% of the population uses some sort of corrective lenses. 30% are near sighted-can see things close but not far away. 60% are far-sighted-they have trouble reading or texting without glasses.