illuminating science

Well, continuing with posting my detailed medical history on the web, I went to the optometrist this morning to have an eye checkup. It was all free (wow! Medicare (Australia’s public health system) is working right! It’s not perfect, but I have to say it beats the pants of the U.S. system.) Right up until the bit where I bought new glasses, anyway…

I have a slight astigmatism in my right eye, which basically means my eyeball isn’t perfectly spherical. I need a -2.5 dioptre lense in the horizontal direction, but an extra 0.75 in the vertical. My left eye, however, is a steady -2.5D. These measurements refer to the strength of the lens needed to correct my eye, and if you take a first year physics course you’ll learn all about them. They’re measured in “inverse metres”, which means that -2.5 diopter is “equivalent” to about 40cm (though what that physically has to do with your vision is a little more complicated.) Basically, as a short-sighter, my “far point” (furthest distance I can see) is at about 40cm or so, rather than at infinity like a normal person. My glasses shift images from infinity to about 40cm.

What I found interesting was that a person with short sightedness (e.g., me) has perfect vision up until their far point - I’d always thought my vision was always flawed, but just negligibly so up close. Of course when thinking about the physics of it, it makes sense - my lens just can’t relax enough to focus those far images, but once I get in close it works just like anyone elses. Another interesting tidbit is that I’m (supposedly) unlikely to ever need reading glasses (or at least not until I’m older than normal) as my eyes are typically more relaxed in close. (I haven’t really thought through the physics of this…)

Lenses also magnify (or diminish?) the images slightly - not much, but when you get a new pair of glasses your world looks a little strange for a few hours. Your brain adjusts pretty quickly though! A neat story the optometrist was telling me was that when he started out he tested his brother. When he asked “Which looks better, A or B?” the brother kept giving different answers. The optom thought he was being made fun of, until he realised that his brother had a major astigmatism (oval eyeballs) in both eyes! When they gave him glasses, he said that everyone looked short and fat, because the correcting lenses basically just squished everything in the vertical direction. Of course, his brain adjusted quickly, but it raises some interesting questions about perception and vision. I particularly reminded of the experiments where people where headsets that flip the world upside down - they learn to adjust within a day or two and notice nothing out of the ordinary. But when they take the headsets off, it takes them another day to adjust to normal!

Who knew a trip to the optometrist could be such fun?!