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Thursday, 21 August 2014

A Curious Optical Illusion Caused by my Spectacles

Sat in a presentation a few days ago I started to get distracted by an odd effect outside. I was in an upstairs room sat on the other side of the room from the window. Outside was a golf course. I was looking at the tiny white dots on the green, presuming they were gold balls but unable to work out why they looked so small. Later on a miniature golfer landed a shot in a bunker. He recovered nicely, tidied up the sand with the miniature rake that had been left there, and moved on. Later still other miniature golfers came and went; some even had a miniature golf buggy.

During a break I wandered over to the window. Now that I could see the drop to the ground, everything looked the right size. It seemed that I had been misjudging distances, thinking things were nearer than they really were, so that they appeared shrunken. When the presentation restarted, I tried to make the golf course look the right size by sheer force of will. It didn't work.

The answer came to me half an hour later, when I happened to take off my varifocals—suddenly the golf course was looking normal. And when I'd stood up earlier, naturally I'd removed my glasses. At this point I confess to missing a bit of the presentation while I experimented pushing my glasses up and down my nose. There was no obvious magnification going on, and although the varifocals do make distant objects appear slightly higher up, I couldn't see how that could be confusing my brain in this way.

I think I worked out what was going on when I noticed how a bit of rough grass changed when I was using my specs: a blur of light green colours changed into distinct blades of grass. My theory is that because the glasses made things more distinct, my brain was fooled into thinking they must be much nearer than they actually were. The only thing that bothers me is that I've been wearing these specs for almost a couple of years now (note to self: book annual appointment at the opticians), but this is the first time I've noticed this effect. My guess is that it was a particular combination of circumstances that generated it, namely having in my near view something (the rough grass) that changed so dramatically when I was using my lenses.