Took me about 10m total to get it all the way to impossible mode. I think you can do it!

...except as you say, it didn't work. The "eye-cross" trick gave the answer on the impossible one in ~10 seconds.

The impossible one was quite tricky, but I did find I was able to relax into the image and take my time. Probably took about 10 seconds.

The impossible one was sub-2-seconds for me. I had to do it over to make sure it wasn't more than one difference...

Makes you wonder if the kid he was talking about had a lazy eye or crossed eyes or something.

Fun fact- when I was a teenager, my friends and I set up a stand in a local mall selling those “magic eye”posters. We made bank for a few months. But, there are actually a lot of people that medically cannot use the technique, or at least for whom it is extremely difficult or less vivid. Severe astigmatism, (obviously) blindness in one or more eyes, and certain attention deficits or fidgety types often have a difficult time.

I, on the other hand, 37 years later,am basically permanently crosseyed from the experience lol. It somehow became a resting state for me from all of the practice, so I’m always doing it on any kind of repetitive patterns, and even “successfully” on random ones which does some really weird stuff in your visual cortex.

How bad does your astigmatism have to be? I've only ever been able to get one magic eye poster to work for me in my life, and I had no idea astigmatism had any impact until just now! I don't know if mine counts as severe, but this would explain a lot for me.

As it happens, I also can't focus on the images in TFA after crossing my eyes to get the shimmer the author refers to.

My daughter has a mild astigmatism (enough to where she needs to wear contacts) and can do the magic eyes very quickly… but maybe that’s only when she’s wearing contacts

I don’t know for sure, but it would make sense if it had to be fairly severe. If it were not enough to meaningfully distort one eye vs the other it would not seem to be detrimental. It’s only when the two images can’t be correlated any more that the problem shows up.

In my recent years I have developed some astigmatism, but I can still do them without difficulty.

Same here. I can get the images to overlap; it feels like a stereoscopic image that lies a bit beyond the screen. But the differences don't pop out to me. I have less trouble finding them looking normally.