cataracts, bandpass filters

impossible.

It's the same process that makes us so much stronger in our 20s than in our teens--it just doesn't stop when we'd like it to. Like it or not, we aren't citizens of this world. (Fortunately, we are citizens of another one, if we choose to be.)

Cheers

Phil Hobbs

Some kids have to wear glasses even though they don't feel the need, but because their eyes are different and they might see double or one is near sighted etc. if it is bad enough the brain just stops using the worst eyes and it will eventually just stop working more or less

-Lasse

impossible.

That's about when mine turned. In my 20s and 30s, my eyes were 20:10 or better. Just after I retired from IBM (at 54) I started wearing cheaters. Within a few months I went to an optometrist and was in bifocals for reading. My eyes changed fast. Shortly I'll be back to the OD (as soon as I can find a new one) and I'll likely get a prescription for distance, too.

Sometimes you're eat the bear and sometimes the bear eats you.

Aren't both images in that scenario focused? With differing R/L focal length that's not going to happen in the wild.

How do contacts differ from glasses?

han

e-

I don't really know how it works, since I haven't gotten it to. I read a description, but don't remember the explanation.

I've tried it with and without glasses, of various working focal lengths, etc.

in

1. Contacts don't minify or magnify the image incident on the cornea. Spectacles do. If the difference exceeds a diopter or two, your brain can't fuse the images and you'll see double. Less difference than that can cause eyestrain.
2. Contacts don't translate images the way that external lenses do when the image is off-center.

(Experiment: take off your glasses and view a vertical line through a lens, as you pass the lens from left to right. The line is translated (prismatic distortion) everywhere except at the lens' center.)

1. Simple lenses have all sorts of horrible off-axis distortions. Eyeballs (with or without contacts) avoid that by looking directly at the target, but spectacles can't (unless you turn your head, and center the image on the lens' axis.) Eyeballs are also not simple lenses--they're aspheric, and marvellous.

The eye's vergence and focus mechanisms interact--if your eyes point inwards, it makes you focus for near. If you focus for near, your eyes try to point inward.

So, if an external lens translates an image, it really confuses things--by altering your vergence angle, it throws off your focus. You compensate by re-focusing, but then your eyes try to point in the wrong direction. You override that with voluntary effort, but the involuntary control fraction activates your opposing extraocular (eyeball pointing) muscles, and they fight you all day.

The result is eyestrain, or double vision.

As mentioned, people's tolerance varies widely, as does their adaptability.

Great idea, good enough for most applications, but they won't do anything for astigmatism or bifocals.

More:

Variations on the same theme:

a.

I was lucky. One eye is short-sighted and the other is astigmatic. In the range where binocular fusion is useful, both were more or less equally bad, and I can do binocular fusion, even through I didn't get glass until I was in my teens - the medical advice my parents got was rubbish.

These days I use Zeiss Varilux bifocals for everything except playing hockey - the lenses are too expensive to risk, and close vision isn't much use in field hockey - and driving long distances when being able to tilt my head eases the strain on my neck.

But I do take them off for very close work - the short-sighted eye focuses brilliantly at around 20 cm.

Bill Sloman, Sydney

Now you sound like Obama; I didn't do it - nobody saw me - you can't prove a thing. ;-)

But again, both images are in focus which won't happen if you intentionally focus each eye at a different distance.

The image isn't "incident on the cornea". The image is formed on the retina. All the lenses in front of that are compounded.

OK, that I can buy. The eyeball moving (contacts) is the corollary of moving the head (spectacles).

The distance from the eye to the lens has changed, too.

That I can buy. The brain takes care of a lot of that, though I do get some "surprises" in my peripheral vision due to the "correction". Sometimes I think something is heading directly for my temple (and my wife isn't even around).

But the compound lens doesn't change, at least on-axis. The real marvel is the brain behind the eyeball.

The "look through" 3D images wouldn't work if that were the case. I can (could do a lot more) certainly relax and defocus my eyes without them turning in or out.

Sure, but I don't think I was talking about that.

Certainly.

er

t than

uble-

al

I don't think it's possible to focus eyes separately, so I'm not sure what you mean.

My eyes are naturally set at different distances; to get them both focused on the same target takes external lenses to compensate the difference in focal lengths.

I've tried the MagicEye diagrams with bare eyes, and various combinations of lenses. It didn't work, it was boring, and that's all I can say. Maybe I didn't try hard enough.

ng in

ges

rk

My lax terminology aside, concave lenses minify images, convex magnify. If the image size is too different between eyes, the brain won't fuse the images--double vision. It's an empirical fact.

Optical path length typically varies in eyes of different focal length--that's a factor.

You can do the same experiment looking over the top of your lenses, if you prefer. The result is the same.

You can also rotate the frame 20 degrees or so, and see how that makes your eyes tug, wander, struggle to track, and sometimes produce double vision.

Brains map the vision space, adjusting vergence / accommodative demand. That's why it takes kids a while to adapt to new lenses.

It is the case. My ophthalmology book is packed away; I can't name the nerves from memory, but I could draw the networks for you. It's an analog controller, and that's how it works. Part of the innervation is volitional, part isn't.

Oh, here it is:

The voluntary component allows you to override some--but not all--of the reflexive impulse to accommodation (vergence accommodation).

I can't speak to the 3D

You weren't, but that's why people should try it first, before cataract surgery, to make sure they can tolerate it.

Considering that it is intraocular, it just wouldn't do to go about vaporizing it. Something else must be going on, but ask the doc, 'cause i don't know.

?-)

You are not supposed to lose focus, but adjust eye aim to a further distance, kind of going "wall eyed" (the opposite of cross eyed). I can do it but is stressful to slightly painful. Stupid design because far more people can go cross eyed a bit at will without losing focus.

focal

To a certain extent i have always been able to have some independent control between focal distance and parallax of my eyes. It may be like walking on your hands or wiggling your ears, very hard to learn the first time.

YMMV

?-)

than

double-

I read about an experiment where (undergrads of course) wore glasses that inverted the world. They got used to it.

than

double-

Yes, I once saw a clip on an experiment that showed exactly that. The point was that our vision is inverted already and that the brain learned to correct once and that it can indeed do it again. IIRC it took a couple of days for the brain to relearn with the "inverted" image. After that, it was much quicker to flip back and forth.

Ya, but did the invert one eye and not the other :-) Mikek

As usual, Larkin is misinformed. The experiment was done by _graduate_ students at MIT... while I was there as a student, so more than 50 years ago.

Both eyes were inverted... After a few weeks everything looked normal... right side up. When the glasses were removed everything was now upside down... so the brain is quite adaptive. ...Jim Thompson

George M. Stratton first did it in the 1890s

-Lasse

But apparently MIT repeated it and took credit.

Is he related to Julius Stratton, President of MIT when I graduated ?:-) ...Jim Thompson