My understanding of the human vision system is that focusing is driven by an analysis of image content: your brain analyzes the image it sees, and if it’s blurry, it adjusts the tension on your lens to change the focal length until the scene becomes sharp and clear.
So what thwarts this process when your eyeball is underwater with no air bubble in front of it? Why can’t my eye find a focal length that results in visual clarity? It seems like it doesn’t matter whether underwater objects are near or far, it’s just impossible to get my eyes to see them clearly.
I’m guessing the index of refraction of the water is far closer (than air) to the index of refraction of my eyes’ lenses. Is the problem that my lens muscles can’t possibly adjust to the new focusing conditions (required focal length of lens is outside the possible range), or is it that the required focal length is possible, but so far out of the norm that my brain says “that can’t possibly be right” and refuses to stretch/relax my lens all that far from what it’s used to?
How do animals that split their time between land and water deal with this? Are the eyes of penguins, seals, otters, and other animals able to focus equally well in air and water, or are they optimized for just one of the two conditions?