Greg Hewgill (ghewgill) wrote,
Greg Hewgill

what a difference 6 mm can make

I went to the optometrist a few weeks ago for a regular exam and a new pair of glasses. They were ready last Friday but I didn't get a chance to pick them up until this Monday. When I first put them on, they seemed very different and uncomfortable. The strength of my prescription had reduced slightly, so I expected a little bit of adjustment, but these seemed way off. Nevertheless, I stuck with them and figured that I had just forgotten what it was like to get used to a new pair of glasses.

I think I was slowly getting used to them over this week, and I would probably have stuck with them for another several years, until I saw this map. The important thing about that map is the colors used - a mostly blue background with some red counties. With my new glasses on, it looked like all the red counties were floating a few millimeters in front of the blue background!

I thought about the problem for a bit, staring at that map every which way, and to make a long story short, deduced that the problem must be that my glasses were made with the centers of the lenses the wrong distance apart. This distance should match the distance between your pupils, which the optometrist measures before ordering lenses. The effect is that the red and blue portions of the image shift relative to one another (because red and blue light refract different amounts through the lenses).

An hour after first seeing the red and blue map, I was already back from visiting the optometrist. They measured the distance between the lens centers at 62 mm, and the distance between my pupils is 56 mm. Apparently this makes a huge difference and they will have to remake the lenses. I went home to get my previous glasses, and the distance between their centers is 57 mm. They are going to remake the lenses with a 57 mm center distance. Meanwhile, I am back to wearing my old glasses.

The main point of this post was that I wanted to see whether I could simulate what I saw on the map (the raised red counties) for people with correct vision. I created the following image, which shifts the red areas slightly to the left and right inside the two blue boxes. With your eyes crossed so the images overlap, your eyes will get slightly different information about where the red areas are relative to the blue. The logical conclusion your brain draws is that the red must be closer to you than the blue, giving it a raised appearance.

I am curious to hear whether you can make this optical illusion work. I can see it fairly easily with my old glasses on or off.

Tags: slack
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