The focal point of the eyeball is not in the centre. When examining the details of 3-D perception, this fact becomes relevant - classically, the position of the eye is considered to be a fixed point relative to the head, where the item of interest is really the focal point of the eye which does moves relative to the head when you swivel your eyes, for convergence for depth perception or simply to see something that's not directly ahead.

I first noticed this when I was wearing a coat with a large hood that closed around my face, even covering my cheeks. I noticed that my peripheral vision was blocked, and then I noticed that it was more blocked when I looked to the side. I looked as far left as I could, turned my head so a particular tree was half visible and half behind the hood. Without moving my head, I looked straight ahead and I could see the entire tree, and even a little bit of landscape to the left of the tree. The focal point of my eye had moved away from the edge of the hood and outwards, so I could see further back behind the occlusion. It was on a less dense part of the retina, so I did not see as much detail on the tree - but I could see more of the tree. Quite an unusual realisation; I turn my eye left to see what's to my left better, and I end up blocking it from view altogether.

An exaggerated diagram, where * marks the focal point of the eye:

                        ---
                       /   \
                      |     |
 ---------------------|*    | left eye, looking left
 line of           |  |     |
 sight             |   \   /
              hood |    ---
                   |


                        ---
                       /*  \
                     -|     |
 ------------      -- |     | left eye, looking (almost) ahead
                 --|  |     |
 extra         --  |   \   /
 visibility  --    |    ---
           --      |
         --
       --
     --
   --  line of
 --    sight
The very fact that this diagram illustrates the effect is evidence that this is more a 2-D effect than a 3-D one.

I consider this to be my test to see if I'm trapped in a virtual reality world, an idea that has been explored in some movies. I'll walk up to the corner of a wall, put my cheek against the wall, and check how far around the corner I can see by looking at the corner and looking straight ahead. If I don't see more by looking slightly away from the corner, then I'll know some sinister organisation or evil overlord is toying with my brain.

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