PHOTORECEPTORS
Photoreceptors specialize in detecting light by using pigments to absorb light energy. Most animals have photoreceptors. Rudimentary eyes, called eyespots or ocelli, detect only light. Image formation necessitates a complex eye that is usually equipped with a lens (structure that focuses light on a group of photoreceptors).

As eyes became more complex over time, two essentially different forms evolved. One variety was the camera eye of vertebrates and some mollusks (squids and octopods), and the other was the compound eye of arthropods.

In the vertebrate, the light-sensitive structure is the retina. Photoreceptor cells in the retina are called rods and cones, based on their shapes. Rods function in dim light, enabling us to see shapes and environment. Cones are responsible for the sight that we use during the day, for making out fine detail, and for color vision.

Binocular vision, crucial to distance and depth perception, enables both eyes to focus on the same thing. The specific placement of the eyes can provide a variety of benefits for the animal. For example, zebra eyes are positioned to let the animal see in two directions, while the location of the eyes within the orbits of the hippopotamus is elevated, enabling the animal to see even when most of its body is immersed in water.