How do the eyes of the four-eyed fish allow it to see above and below the water at the same time?

The four-eyed fish (Anableps) is a surface-dwelling fish that possesses specialized eyes adapted for simultaneous vision above and below the water surface. These eyes are positioned on the top of the head and are divided into two distinct parts: an upper half for vision in the air and a lower half for vision in the water.12

To achieve this amphibious vision, the fish utilizes an elliptical lens. The relatively flat sides of this lens add little to the focusing power of the cornea, which is effective in air but ineffective underwater. Conversely, the ends of the lens have a higher curvature, which focuses light entering from below the water's surface. By cruising the surface meniscus, the fish can effectively monitor its surroundings both above and below the water at the same time.2