If you have two eyes, you have double vision. Each eye sees a different view of the objects in front of your eyes.
Try this experiment.
Get two pencils. Hold one pencil about 18″ away from your face slightly to the right and hold the other pencil at arms length slightly to the left. While staring at the closest pencil and keeping it in focus, make an effort to become aware of the pencil in the distance. Hey, there are two of them back there. What’s up with that?
Now look focus on the pencil in the distance and stare at it. No, there aren’t two pencils back there anymore. But wait! While holding focus on the pencil farthest away you now can become aware of two pencils being held about 18″ away (if you didn’t move).
If you are like most people, you never noticed that you had double vision because your brain fixates on the object you are focusing on. Everything else is perceived as a type of background noise that we simply don’t pay any attention to.
Notice also that the pencil that you aren’t focused on isn’t doubled exactly. One eye will be slightly dominant over the other eye and you will perceive one of the doubled pencils with what appears to be a level of transparency that differs from the other one.
Guess what? The farther away you put the pencils the less you notice the double vision. But it is still there out to about thirty – fifty feet at which point the difference is so subtle that you brain can completely mask the disparity and you can’t perceive it as distinct double vision.
For a multi perspective photograph, something radically different happens. Your eyes move apart and closer together as they look at things in the 3D photo that appear closer and farther away, but they stay focused on the surface of the photograph. The suppression of double vision is a different experience because the focus doesn’t change. As the difference increases (close to far away) it actually can become uncomfortable for some people to look at since the brain is expecting to focus the eyes differently for something up close and compared to something farther away. But just like the perception conflict of the inner ear to vision for astronauts floating in zero G, with a bit of practice – and keeping the amount of distance depicted not that great at first – it can be accommodated by most people.
The same is true for motion pictures. If the producers don’t get crazy with the amount of depth they try to create and have stuff appear to come way out of the screen, this taxing of the brain to overcome perception conflict doesn’t have to reach headache levels.