If you’ve ever seen those weird charts of letters like the one here, you know that they don’t spell out words. That poster is an “eye chart,” and eye doctors use it to figure out how strong your vision is. The 8th row of letters is the 20/20 row, which means someone with “20/20 vision” can read it from 20 feet away. If at 20 feet you can’t read those, but can read the bigger letters in the 20/30 row, that means your vision is 20/30: what you see at 20 feet, a 20/20 person can see from 30 feet. If you’re 20/40, that means the 20/20 guy can read the same thing from 40 feet, and so on. You cover one eye at a time to read the letters, because your two eyes can have different strengths of vision. As we’ll see below, you might even find out one of them has superpowers.
Wee ones: If your left eye can read the top 7 rows but your right eye can read 1 more row than that, how many rows can your right eye read?
Little kids: If on a sign you can clearly see the 1st letter, the 4th, the 7th…which one do you think you can see next? Bonus: The big initials on your friend’s backpack might be readable from 7 feet away… but if you have superhero vision that’s 10 times as strong, from how far away can you read those letters?
Big kids: If you have 20/20 vision but your friend has 20/40, that means you can see everything from twice as far. If your friend can read the ice-cream truck menu from 17 feet, from how far can you read it? Bonus: If another friend has 20/60, that means you can read letters from 3 times as far. If you can read the baseball scoreboard from 123 feet away, how close does your 20/60 friend have to stand to see it?
The sky’s the limit: If you have 20/20 vision and your friend is 20/50, from how far can you read letters than your friend can see from 100 feet?
Wee ones: 8 rows of letters, including the 20/20 one!
Little kids: The 10th letter. Bonus: 70 feet away.
Big kids: 34 feet. Bonus: 41 feet.
The sky’s the limit: From 250 feet! Your vision is 5/2 as good as your friend’s (50 divided by 20).
Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking before she could walk, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.