Why can human eyes not focus underwater?

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?

I’m ridiculously nearsighted (e.g. can’t focus on things further than six inches from my face) but I remember being able to see things clearly underwater. My understanding is that it has to do with the index of refraction, as you guess. Just as my eyes can’t possibly focus in air, normal eyes can’t possibly focus underwater. With less difference in the index of refraction between eye and water, a normal eye is actually focusing on a point far behind the retina.

You’ve got it. Most of the focusing done by the eye doesn’t happen with the lens (which is for correction and fine tuning), but at the cornea-air interface. The focusing power of the lens is greater for larger difference in index between the lens medium and what it’s immersed in. Air has an index of 1.003, water has an index of 1.33.

The material of the cornea and of the lens is mostly water, so it’s pretty close to 1.33, so when you’re in water the possible power of the cornea-water interface is much lower than the cornea-air interface has. The difference is much greater than the lens in your eye can correct for. Hence, everything looks blurry underwater.
There are basically two ways to fix this – you can trap some air near your eyes so that you look through an interface with air. This is what you do with goggles (you can also, as I did as a kid, hold your hands around your eyes, blow air into it, and use the impromptu bubble to see). Or you can make corrective glasses for underwater viewing. These have been proposed and built. They look like “negative” lenses (thinner in the middle, thicker at the edges), filled with air.

So how do animals (penguins, seals, etc.) manage to see well in both environments? Walruses are bottom-feeders, so they get by just fine underwater by feeling their way around with their whiskers and tusks. but penguins and seals need to be able to see well underwater to target fast-moving prey, and they also need to be able to see well on land to navigate over/around various obstacles and hazards. Are their lenses made of substantially different material with a higher IoR, or have they just developed lenses with a much wide range of adjustment?

An excellent and non-trivial question.It’s even more complicated, because some animals look above and below the water simultaneously, and have to be able to focus while looking through both air and water interfaces at the same time. And I don’t know the complete answer. I will point out that you can locate a large target without being able to focus on it perfectly. I can see a large ball underwater, for instance, even though I camn’t see the details.

Regarding birds:

I’ve heard that some species that see both in and out of water have bifocal lenses, with the bottom half of the lens bulging out significantly, while the top half is much less strong. So if they’re right at the surface with the waterline across their eyes, they’re getting sharp images from both parts of the eye.

External air pressure vs water pressure. A rubber bladder filled with air at sea level will decrease in size relative to the number of atmospheres the bladder descends in the water column. A rubber bladder attached to a metal cylinder will withdraw into the cylinder as the water pressure increases. Increasing the air pressure in the cylinder will push the bladder out and even cause it to expand depending on how much the internal air pressure is increased.

Assuming there is a lens on the end of the bladder, the focal plane will change as the bladder changes shape.

As CalMeacham alludes to, trapping air near your eyes allows your eyes to focus naturally under the pressure of the water. Goggles and face masks work because the exposed part of your eyeball isn’t being distorted. The human body is basically a water tight cylinder (if you keep your nose and mouth shut). The unexposed part of the eyeball is protected from the increases in water pressure.

I assume that mammals capable of spending 10 minutes or more under water either have the ability to adjust their internal eyeball pressure to maintain their vision or they have tougher eyeballs that can retain their shape. My money is on presure regulation.

I find this a very weird response. No animal I’m aware of uses pressure to change the shape of its eyeball or in any other way adapt between air and water vision. It is, I suppose, a possibility, but no creature has used any variation of this.

What? Your body, being mostly water, is at the same pressure as the surrounding water. Diving any distance below the surface causes the air spaces in your body to contract due to the surrounding pressure, unless you introduce more air into them. That’s why your ears hurt when you dive deep.

Scuba masks are designed so that your nose is inside the airspace, allowing you to add enough air to maintain the same pressure as the surrounding water. Neglecting to do so results in mask squeeze, where the flesh of your face and eyes get sucked into the mask - a rather unpleasant sensation. Swim goggles cannot be used for diving for this reason - try diving to the bottom of a 12 ft. pool with goggles and you can definitely feel the squeeze. But when you’re wearing a mask and at 33ft. depth, all parts of your eyeballs are at 2 atm pressure. Which still has nothing to do with focusing.

Saw a show about mermaids that show some islanders that can focus their eyes at will, even underwater, don’t know which island or if it is true though.

“Mermaids: The Body Found” ? If you dig around a bit on the Discovery website, you can find a statement that the following two things from that show are true:

Whale beachings due to high power navy sonar.

The Bloop.

Everything else is fiction. Including the spears found in the fish, and the islanders contracting their pupils at will so they can “hunt” underwater.

I suppose that if you look through a pinhole you coukld see underwater – pinhole vision allows you to see good images regardless of index difference*. But you’ll lose an awful lot of light.

*The Tridacna Giant Clam has a pinhole in place of a lens on the eyes around its mantle. the Nautilus, the shell-bearing cephalopod that is the namesake of Captain Nemo’s sub and of the exercise equipment using a cam resembling its shell, also has a pinhole eye.Both the clam and the Nautilus have rotten resolution, though.

As others above has posted, the pressure differential isn’t the significant factor. Otherwise, there would be an enormous difference between vision just below the surface (at nearly 1 atmosphere) and 33 feet down (2 atmospheres). The deepest I’ve been is just over 100 feet (4 atmospheres), and my mask worked just fine despite all that pressure. Taking off the mask at 100 feet has the same effect it does at one inch, visually.

The reason for this is that the eye is filled with liquid, most of which is water, and which has about the same compressibility as water. There are no air spaces in the body associated with vision. However, there are air spaces in nasal cavities, to which divers with sinus issues can attest!

(Those air spaces should communicate with the mouth/lung system. When they don’t, you get nasty headaches.)

I’m not an expert on mammal eyeballs. It was just an assumption that I’ve had. A harp seal can dive some 275 meters for as long as 15 minutes. It must have some way to adjust to over 26 atmospheres.

I wasn’t suggesting that saltwater mammals changed the shape of their eyeballs but that they, in some way, prevented water pressure from changing the shape of their eyeballs Altering the shape of an eyeball alters it’s focal plain and affects vision.

Please correct me if I’m wrong, but your vision changes when you put your face into water without eye protection. Wearing goggles prevents water pressure from altering you eyeballs shape and you can see fairly well depending on the available light. A diving mask allows you to adjust the pressure inside the mask as you dive deeper. Goggles and masks seperate the exposed part of the eyeball from the pressure of the water.

At 33ft depth, if all parts of my eyeball are at 2 atm, why would I need a mask?

For the same reason you need it when your eyes are an inch below the surface.

While that reason must be obvious to you, would you mind sharing it with me?

When water and it’s associated pressure is in contact with the exposed part of the eyeball, vision is distorted/altered. When the eyeball is protected from direct water contact, vision is OK.

Because as CalMeacham explained in post #3, the human eyeball is optimized for an interface with a fluid with a refractive index of 1, like air, and not a fluid with a refractive index of 1.33, like water.

Like Chronos said above - you need a mask even when your eyes are 1 mm underwater, even though the pressure there is only 1.0001 atm.

No matter what you do, besides using a rigid submersible, all parts of your body will be at the same pressure as the surrounding water at all times. If you don’t do something to equalize the air spaces, which will compress under pressure, then the soft tissues of your body will compress those air spaces for you. Which hurts.

Thanks. It’s a refraction issue. Got it. Ignorance fought.