Thursday, 16 October 2003

My Opia

Last time I saw an Ear-Nose-and-Throat specialist, he showed me how to read the CAT scan that I'd had, pointed out where the abnormalities in my sinusses were, and cheerfully concluded that I was a Mutant.

Well, I knew that.

I can still hear sounds of over 19 Khz, when most people lose the ability to hear much over 15 by the time they're in their twenties. And my eyes are sensitive to the near Ultra-Violet, so I can see patterns in Daisies that other people can't, and can see really well on cloudy days. This last condition is surprisingly common.

But it has its downsides. My eyesight has been visibly (sorry) deteriorating over the years, and it was about time I visited an Optometrist. He didn't take long to make a diagnosis. -1.25, -1.00 x 75 in one eye, -1.25, -1.00 x 105 in the other. Still safe enough to drive without glasses, but not by much.

I was intrigued by the prescription. What do those numbers mean? Well the first one (-1.25) says that I have mild myopia. Things at less than 1 metre I can see really well (useful if you're spending most of your day at a computer screen, like I do). But at 2 metres, there's a barely noticeable difference, and at 20 metres, a noticeable one.

The second number (-1.00) says that I have mild Astigmatism. That means that I see things wider than they really are. The other numbers specify the orientation of the distortion of my cornea.

There's a good simulator of myopia and a simulator of astigmatism on the web. Adjust the slider to about mid-way between "normal" and "moderate" to see what I see without glasses.

Of course normal people get less myopic as they get older. In fact, many get so long-sighted they need reading glasses. I have the opposite problem. *sigh*

Now for a swift digression on Optics and the way the Brain works: Examine the picture below : blind spot test

Watch the bee travel from the flower to the edge of your screen.

Now close your LEFT eye, and line up your right eye so it's looking straight ahead at the flower, about 30 cm (1 ft) away.

Keep looking...

The Bee will disappear at one point, and reappear later.

This is because of you "blind spot", an area of your vision where you can't actually see anything - what happens is that your brain automatically "fills in" the missing area by just extending the things around it. In this case, the bee vanishes, and the brain uses the grey line to fill in the place where the bee is.

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