I always experienced them 'back-to-front' as well, and I believe it is what happens if your eyes are focusing on a point in front of the picture ('cross-eyed') instead of a point behind it.
With a lot of effort, I can see it the right way by putting the image very close to my face, and relaxing my eyes to focus on an imaginary point beyond the page/screen. Then I slowly back away so that the image fits more comfortably in my field of view. However, I still find it very hard to bring the image into sharp focus, and at any moment the illusion can suddenly disappear and I have to start again.
I have excellent vision but after trying to look at these stereograms for a few minutes I am suddenly having difficulty focusing on normal objects around me. Quite unnerving.
With a lot of effort, I can see it the right way by putting the image very close to my face, and relaxing my eyes to focus on an imaginary point beyond the page/screen. Then I slowly back away so that the image fits more comfortably in my field of view. However, I still find it very hard to bring the image into sharp focus, and at any moment the illusion can suddenly disappear and I have to start again.
I have excellent vision but after trying to look at these stereograms for a few minutes I am suddenly having difficulty focusing on normal objects around me. Quite unnerving.