Meet the Anableps the four eyed fish


Picture of anableps the four eyed fish.

Bifocals—Who had them first?


Two centuries ago, Benjamin Franklin took two pairs of glasses, one for distant vision and one for near vision, and cut each in half horizontally.

Then, in the same frames, he mounted the upper half of the distant lenses on top of the lower half of the near lenses, and behold, the first pair of bifocals!

Today, advanced technology has made it possible to make bifocals from just one piece of glass, with curves of different strength in the top and bottom parts.

And there are even bifocal contact lenses.

But did you know that long before optical science developed bifocals, an obscure freshwater fish was wearing ‘the latest’ in one-lens bifocals?

You will find this foot-long called Anableps by scientists, in the waters stretching from southern Mexico to northern South America.

From tail to gill, it is undistinguished in shape, but beyond the gill, it is eye-popping.

At first glance (as the picture above shows) these fish appear to have four eyes—one pair looking up and one pair looking down—prompting people to call them four-eyed fish.

But that’s an optical illusion.

They have two big round eyes, but each eye is divided horizontally into two parts by a band of skin.

Since these fish swim along the water’s surface, the upper halves of their eyes are like periscopes that protrude above the water and scan the sky, while the lower parts remain submerged and look underwater.

In this way the four-eyed fish searches for food below and at the same time keeps an eye—or, better yet, two eyes—out for hungry water birds above.

To look underwater, however, the fish needs a thicker lens than it does to look through air. 

How is that problem solved? Bifocals!

Each eye comes with a one-piece, oval-shaped lens that is thicker at the bottom than at the top.

So anything viewed underwater is seen through the thicker part of the lens, while the flatter, upper part surveys the sky.

But the two-tier vision of the fish will only be clear as long as they keep the lenses clean.

How do they clean their lenses?

Whenever the lenses dry out, the fish simply duck their heads underwater and emerge with sparkling bifocals again.