To me, it's no harder than "snapping" to the central image when wearing VR glasses like the Vitures. What works for me is telling myself that the central image is further back than the two slightly unfocused images either side. Just keep telling yourself "it's further away, and it's the real one. Those two are fake." As soon as it clears up just a little, I then allow my gaze to wander around the image. It's extremely important to try this on a real monitor because small movements to the phone in your hand might get in the way. Or in my case, I'm longsighted in one eye, so while my left gets a clear image my right doesn't. I have to use a large image on a real monitor at least 2 feet away.

Another method is parallel eye though it's bit more complicated than the finger.

Zoom the images in front of your eyes so that they are little less than same width as your eyes.

Try to look behind/beyond the image, as in let your eyes loose/relax. Don't stare at the images instead see in that direction as if you are day dreaming.

Images will overlap at one point. If they don't completely perfectly overlap, reduce the zoom.

Once they overlap, focus the overlapped image.

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Method 2: Use kitchen towel rolls

After putting those images in front of you, make a binocular out of used kitchen rolls.

Point each roll to each image.

There you go.

Does it not work with glasses?

I was having this problem as well, but I kept trying, and then I got it. I found the finger trick was useful to initially sort of calibrate the focal distance but overall didn't really help me that much.

Here is what worked for me. I used my laptop, zoomed in a bit on the images and brought the screen fairly close to my face. I ensured that the image was crisp using each eye (I also have astigmatism, and I probably also need reading glasses, but there is a sweet spot where both eyes have good focus, and I ensured I was there.) While crossing my eyes a bit, I start to see a third image in the center of the two images, but it's either out of focus (like two overlapping images), or it's very thin, like it's not the full image. I relax and keep my attention on this imperfect image and try to focus on it without trying too hard. Using this approach the image suddenly comes into focus and I no longer have to try to keep it there.

I feel like the key might be to notice the very beginning of the desired image in the center and then to try and focus on it, but in a bit of a relaxed way.

Incidentally when it works it is extremely weird! The other images essentially disappear and it's like you've travelled to another dimension.

You may have a very slightly 'lazy eye' (I do) - it can be a lot less extreme (not at all noticeable to others) than the pointing-completely-different-directions that people imagine, and iirc is highly correlated with astigmatism.

Optician used to tell me to work the muscle by following my finger to my nose, trying to maintain a single image. At a certain point it will snap into two - the 'lazy' eye has given up and drifted slightly - the goal is to get the finger as close as possible. Obviously if you get very close or all the way, that's 'cross-eyed', but I just can't do it.

What's more a lot of people (maybe 20%) don't benefit from things like

https://www.reald.com/

which is one reason why stereo movies have struggled. (That plus some people get sick... Having both a flat and 3-d movie in two different theaters comes across as money grubbing to the consumer but it is really a money sink to the theater.)

For me, what's difficult is holding my right eye closed without my left eye drifting to look at my nose. My right eye's good, I can move it and focus on anything within my (now peripheral-limited) view... but the left is wonky. I think I learned how to wink (and hold it) with the right really early, by age 3 or 4, but the other side I never tried until I was pre-teen... some sort of muscle atrophy?

You can also tell if your head's level, just by crossing your eyes. If the two images are diagonal to each other, then your eyes/head aren't level. I have no idea what the possible use for that would be.

Which one makes things become bigger? I learned that one first and then later figured out the one that makes the mixed image smaller (cross eyed I think?). Now I cannot do the big one anymore.

Thanks, that is nicely explained --- you finished the thinking I had only started!

Are the eyes mechanically capable of pointing outward (so the interpupillary distance is not longer a constraint)? If so, is the problem then neurological not mechanical (brain doesn't want to send signal so they do that)?

Ever since I was a child addicted to the "magic eye" stereogram books, I've always diverged (not crossed) my eyes for spot-the-difference puzzles.

Also, if you're doing it on a piece of paper, hold a pen in each hand spaced right so you see the middle (3rd) hand in the middle combined image, and move both hands in sync to circle all the differences. Kind of a cool way to point them out to someone else.

The difficult puzzle took me about 10 seconds here since I was looking for more than one difference. I saw the first difference in about 1 second.

You can easily generate inverted ones that require crossing your eyes to appear properly, but they don't look as nice since they pop out instead of going into the screen/book.

Better known as Magic Eye, but yes.

Crossing is easier because you can simply hold your finger in front of your eyes and look at that for practice.

Diverging requires you to look past the image, meaning you have nothing to really look at, which makes it difficult to figure out what your eyes are even supposed to do.

Those stereograms aren't helping much either, since they look like nothing until you get it right. With cross-eye you have instant double-vision that you just need to align.

Cross-eye also works across much larger distances, diverging fails when the images are too far apart.

It depends on the image. If the two images are too far apart then it could require your eyes to diverge, and not to just converge slightly less. That might be impossible.

Diverging is way way easier for me, but I am positive that's because of the 10's of hours (at least) that I spent staring at magic eye images as a child.

For crossing just focus on your finger and then remove it.

Looking far away may be harder, and afaik it’s near impossible to look “past infinity”, iow pictures must be less wide than the distance between your eyes.

Btw these two methods aren’t equivalent in watching stereograms. If you look at one and see something but it doesn’t really make sense, then it’s probably the opposite chirality.

Personally I hate the crossing method because it makes your eyes feel strange for a while.

how I approached crossing: first practice just crossing your eyes and observing how every object has two images in this case and when you slowly “uncross”, they merge back into one. you can use anything in your surroundings.

then for the stereogram you do the same, observe the out of focus edges of the left and right pictures, then slowly uncross until left and right image occupy the same spot as though they were the same object. now its out of focus, but one (ok, actually three, because there were two, you “doubled” that by crossing, then merged two of them. but ignore the other two and focus on the merged pair)

sometimes you will merge images of the same picture, in this case you are just back at your normal vision, repeat :)

then you try to keep them overlapped and focus the vision, try to “believe” that you are really looking at a single object.