[Les]

Thanks for the feedback.

Having played around with these images a bit now, I've found there are a couple of important factors in avoiding eye-strain while viewing and making them.

As the end user/viewer, you must have your eyes horizontally level with the picture, and you must have your eyes the right distance from the picture in order to allow your eyes to just relax out of focus and not strain to make the out of focus pictures merge. If you move your head back and forth you can use that as a focussing method and see how close you can get to the pictures before they become too big for your eyes to merge comfortably. I find I have to move further back from the screen than usual when viewing the images fullscreen on Youtube for example.

As the picture-maker, you have to make sure the two separate images are identical in terms of their content (which can be hard to do when the Cliffs Of Dover replays often don't generate the same objects and effects on consecutive plays), and identical in terms of being in synch with each other if they're moving pictures.

Beyond that, I guess you've just got to make sure you don't overdo it and strain yourself, just like anything really.