Sharks have such keen senses that enable them to detect even small amounts of electric current or vibrations (electroreception) and chemical changes in the water (chemoreception). These senses are so acutely specialized that it was assumed that they overcompensate for sharks’ poor vision.
In the late 1960s, researchers discovered that shark eyes have duplex retinas, or retinas containing both rod and cone cells. Rods enable the shark to see light and darkness, while cones allow for the detection of color (though scientists are still unsure how sharks interpret colors). The eye structure of a shark is similar to other vertebrates in that it contains a cornea, lens, retina, pupil and iris. Given their similar structure and usage, shark eyesight works similarly to that of humans.
Unlike humans, shark eyes are equipped with a tapetum lucidum, a layer of mirrored crystals located behind the retina. This provides a means for light that initially escaped detection to be detected as it is passed through the retina a second time. This process defocuses light, reducing acuity but increasing sensitivity. That is, at least you can see, but you don't see as clearly as you would if more light were available and you didn't have to use a tapetum. Such an adaptation enables sharks to see even in dark or murky water and up to ten times greater than humans in clear water. Whereas human eyes are adapted only for land and cannot see in the water, sharks have keen eyesight in the water and may even see out of water.
Also unlike humans, shark eyelids serve to protect the eye when attacking prey. Some sharks have a nictating membrane, or a clear membrane that covers and protects the eye when a shark bites its prey. Sharks like the Great White, lacking a nictating membrane, roll their pupils back in their heads for protection when feeding.
Also unlike other animal orders, the size and function of the eye can vary greatly amongst shark species. Sharks are uniquely and amazingly adapted to their in environments in every way including eyesight. Lisney and Collin (2007) conducted a study on the absolute and relative eye size of 46 elasmobranch species. Shark eye and pupil size varied depending on the layer of ocean in which they predominantly reside.
Sharks living in the pelagic (0-200m below the surface) and upper mesopelagic (200-1000m) had the largest eyes presumably because they live in areas with the most available light. In the Bathylpelagic zone (1000-4000m) shark species have smaller eyes with larger pupils so as to increase visual acuity and ability to detect bioluminescence. The feeding habits of sharks also influence eye size, whereas sharks who feed on larger, fast moving prey have larger eyes than sharks who primarily feed on sedentary prey.
Despite their vast sensory abilities, sharks still rely on eyesight when seeking food, therefore, their eyesight is as keenly adapted to their environments as their other senses.