# Why can human eyes not focus underwater?

**URL:** <https://boards.straightdope.com/t/why-can-human-eyes-not-focus-underwater/631371>\
**Category:** Factual Questions\
**Created:** [August 13, 2012, 12:42pm UTC](https://boards.straightdope.com/t/why-can-human-eyes-not-focus-underwater/631371 "2012-08-13T12:42:24Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\
**Post date:** [August 14, 2012, 2:17pm UTC](https://boards.straightdope.com/t/why-can-human-eyes-not-focus-underwater/631371/21 "2012-08-14T14:17:14Z")

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> [@Chronos](#):
>
> 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.

Here’s a cite on those fish - [Four-eyed fish - Wikipedia](http://en.wikipedia.org/wiki/Four-eyed_fish)

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**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\
**Post date:** [August 14, 2012, 3:14pm UTC](https://boards.straightdope.com/t/why-can-human-eyes-not-focus-underwater/631371/22 "2012-08-14T15:14:03Z")

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> [@doorhinge](#):
>
> 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.

It doesn’t need to adjust for pressure, since changes in pressure don’t change the shape of the eyeball enough to matter.

> [@](#):
>
> 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.

See above.

What’s the difference in compressibility between aqueous humor and water?  
What’s the difference in compressibility between vitreous humor and water?

Based on my experience to 100 feet, the differences are negligible. Note that the difference between 0’ and 33’ is twice the pressure. If pressure significantly effects vision, it would be clear by this point.

For example, pressure affects boyancy. There is a huge obvious difference in boyancy near the surface. As you go down, you have to inflate your boyancy compensator considerably. The deeper you go, the fewer additional adjustments you need to make. The same would be true for adjusting for optical differences due to volume changes caused by pressure changes.

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