I recently had LASIK procedure to correct my myopia (i.e., nearsightedness). Prior to that, I learned a few fun facts about eyes and prescriptions for eyes.

Visual Acuity

When people think about their vision, they often think of it in terms of a comparison to an ideal “20:20” vision. But, really this just means “normal” vision, not necessarily perfect or ideal. It’s a ratio that compares what you can resolve (the level of detail you can see) to an average resolution for humans, in general.

The first number in this ratio is what you can resolve at a distance of 20 feet. The second number in this ratio is the distance that an “average” person can resolve the same level of detail. In other words, if your vision is 20:20, you can see at 20 feet what the average person can see at 20 feet. If your vision is 20:40, you can see detail from 20 feet away that the average person can see from 40 feet away.

If your visio is 20:200, like mine was, that means that what an average person can see from a distance of 200 feet, you need to be 20 feet away to see. Yikes.

Sinister Eye

If you have a prescription for glasses or contacts, you might notice that there are two measurements, one labeled “OD” and the other labeled “OS”. This stands for “Oculus Dexter” which is the right eye, and “Oculus Sinister” which is the left eye. Latin people were weird about the left side of things.

Side fact: Being “dexterous” means being right handed. If you’re left handed, you’re technically “sinistrous”. 

Lonely in Blindness

Poor vision is generally a genetic trait, but sometimes it can result from a physical injury. Neither of my parents, nor brothers, nor first cousins, nor aunts or uncles, nor grandparents (in their youth) required corrective lenses, but I was nearly legally blind by the age of 10. Why?

Turns out, when I was born, my head was so big that my mother couldn’t squeeze me out without some assistance*. The doctor who delivered me used forceps to pull my head out. You can actually see the outline of the forceps as indentions in my skull, which terminate on either side of my face, just beside my eyes. The forceps affected the shape of my skull just enough that it laterally compressed my eyes, elongating them and causing my myopia.

If you’re reading this Sidney, you inherited your need for glasses from your mother’s family. She and both her parents require corrective lenses. You were doomed from the start. Sorry…

* An alternative explanation for this is that I had some sort of prenatal precognition and I could sense that the world was a cold, dreary place and wanted to stay in the womb forever.