eli5, how is it that cameras can see clearly underwater whereas human vision gets blurry?
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You get a lens when two materials with different indexes of refraction meet along a curved surface. For the human eye (or any land animal’s eye), the two materials are air, and whatever the eye is made of (mostly water). For a camera, that’s glass and air, plus more glass and air inside the lens because a camera lens almost always contains multiple pieces of glass.
Put a curved eye under water, the index of refraction is all wrong for what the eye evolved to handle, and you’ll never get it to focus.
Creatures that live under water evolved eyes that work when in contact with water. I presume a fish would be just as blind when pulled up into the air.
You can avoid the whole problem by having the interface between the eye and the outside environment be completely flat. Light passes through an interface like that without bending. Put the actual lens parts behind the flat surface.
This is why you can see clearly into an aquarium, or through swim goggles or a scuba mask. Underwater cameras are either placed in waterproof cases with a flat piece of glass for the camera to look through, or if the camera is truly waterproof like a [Nikonos](https://en.wikipedia.org/wiki/Nikonos), the front surface of the lens will always be flat so that the camera works both in and out of the water.
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