Date: 2004-10-09 00:01:00
Tags: slack
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.

Ahhhhh! You snow-crashed me!
Worksforme. :)
Bizarre. I'm completely incapable of merging the dots/boxes into one, but I can reliably turn them into three dots/boxes.

My brain hurts.
OK, now I have it I think (after some coaching from equiraptor).

The red looks like it's a bit behind the blue, though.
It's possible that your eyes and/or glasses are a bit different and see things differently than mine. That's interesting.
When you're crossing your eyes, are they moving away from your nose, or towards your nose? If your eyes are moving outward, the effect is reversed.
I'm not even sure I know how to tell. But this certainly does sound like a plausible explanation for the difference.
I hadn't even tried that because I didn't think I would be able to get it to work. But after a couple of tries I made it work. My poor eyes.
your eyes can cross different ways?!?


im with brain hurts...
i got the same thing...all i can get is the three boxes...

bah...i could never do those majic eye things...
Oh, also. This posting of yours is making me feel really deficient and shallow-minded. Here you go creating for us a cool optical illusion and backstory and the best I came up with was a lame Cool Hand Luke moment of "hey! I ate a shitload of carrots".

I blame the beer.
Hey, at least you came up with something. I've been sitting here all day, and do you see any new entries in my journal?
Yeah, but you're on drugs. You have an excuse.
Yeah, and they wore off an hour ago.
Exhibit A:

Exhibit B:

FXO hardware for my Asterisk system arrived on monday and I've been too lazy to even plug it in all week. The content in your posts often far exceeds mine. :)

Then again, I've been working on minilink.
"Signed by: GENTILE"

Gee, that only leaves what, 6 billion people who could have signed for it...
Haha. I hadn't even noticed that.
Okay, here's a new one:

[info]tycoonjack : ahhhhh
this one jumped out at me and started to barrage me with political attack ads
sounds like it's time for a new optometrist
hrm...ya really do learn somthin new everyday...there are centers to glasses lens? (lenses...lensi? for some reason glasses lens jes sounds really wrong...)
Yeah, I knew they measured but I never really realized how important this was until yesterday. This is probably much of the whoa factor you get when you try on somebody else's glasses, even if they have the same prescription.
I can't really get the effect. But then I can almost never do "magic eye" posters either.
[info]robo_cat : First Strabismus Exotropia experience ever for me!
Strabisbus Exotropia has always eluded me - got it this time though! Technique:
1. I put my face very close to the monitor and closed my eyes - let eyes relax into facing dead ahead.
2. Open eyes for short periods, while keeping from focusing too hard.
3. Move head around or in/out (still blinking) until you get three blue squares with the central square aligned/registered correctly for both eyes (still blurred though, central square dark blue, outer squares light blue).
4. Slowly move face away while keeping eyes registered.
5. Eventually get focus!

I have always found cross-eyed magic eye easy, but never managed to do the opposite of cross-eyed.

PS: Nice deduction about distance between lens centres!
Greg Hewgill <>