My eyes are sensitive to light (blue eyes are usually more sensitive) but the stereogram exercise is to exercise the eyes muscles. When we spend a lot of time reading off as screen close by we use a specific range. When looking at magic eye stereograms we are forced into a different mode and I found it helpful for myself. I could not see stereograms at first and had a hard time with a binocular microscope as well, the image would not converge into one. I first learned to use that (and I remember pulling one of the eyes from the side to make the two images converge into one and once converged I would let go the finger and let the eye muscles get used to the new position) then I struggled a couple of days with stereograms. After a day or two of trying I started to make the image pop up but would lose it quickly. After more exercises I could keep the image in focus and look change the gaze all over the image, stare at each corner, etc. When I managed to do that my eyes started to feel much better, the eye strain that had been bothering me for some months went away. The eye strain comes back when I spend too much time in front of the screen and looking at a few stereograms makes it go away. Not entirely sure if you had the same problem but I did hear a bunch of people healing eye strains with stereograms. It could possibly be a certain type of eye strain only. Hope you figure it out one day.