Light rays entering the eye are refracted, or bent, when they pass through the lens. Normal vision requires that the rays focus on the retina. If the eyeball is too long, an accurately focused image falls short of the retina. This is called myopia, or nearsightedness. A nearsighted person sees distant objects unclearly. Farsighted focus, or hyperopia, results when the eyeball is too short. In this case, an accurately focused image would fall behind the retina. These conditions can also occur if the muscles of the eye are unable to alter the shape of the lens to focus light rays accurately. Humans are capable of focusing on a single object with both eyes. This type of vision, called stereoscopic vision, is important since it allows for depth perception. In addition to humans all predatory vertebrates have stereoscopic vision.
Light rays reflected from any object we look at enter the eye and are focused by the eye's optical structures: cornea, iris, pupil, and lens. The final destination of the light rays is the retina, a layer of nerve tissue that lines two-thirds of the back of the eye. In the center of the retina is the macula, an area that is only 1.5 mm (0.06 in) in diameter. The macula is responsible for the clearest, most detailed vision.
The retina is made up of two types of cells: cones and rods. Cones are nerve cells that are sensitive to light, detail, and color. Millions of cone cells are packed into the macula, aiding it in providing the visual detail needed to scan the letters on an eye chart, see a street sign, or read the words in a newspaper.
Cones also produce the sensation we call color. Cones contain three different pigments, which respond either to blue, red, or green wavelengths of light. Cones mix the color signals to produce the variety of colors we see. If a person is missing one or more of the pigments, that person is said to be color-blind and has difficulty distinguishing between certain colors, such as red from green.
Rods are designed for night vision and the detection of motion and objects. They also provide peripheral vision, but they do not see as acutely as cones. Rods are insensitive to color. When a person passes from a brightly lit place to one that is dimly illuminated, such as entering a movie theater during the day, the interior seems very dark. After some minutes this impression passes and vision becomes more distinct. In this period of adaptation to the dark the eye becomes almost entirely dependent on the rods for vision, which operate best at very low light levels. Since the rods do not distinguish color, vision in dim light is almost colorless.
Light rays that reflect from the upper half of any object we look at are focused on the lower half of the retina. Rays from the lower half of the same object are focused on the upper half of the retina. This would seem to give us an upside-down picture of the world. Fortunately, these signals are rearranged when the brain processes them into an image that is right side up.
Another feature of eyesight is stereoscopic or binocular vision, the ability of both eyes to look straight ahead but see the same scene from a slightly different angle. The eyes' visual fields overlap in the center, and the brain merges these images to create a sense of depth important for judging distance. Humans and other mammals have stereoscopic vision. Birds, fish, and snakes have monocular vision in which each eye sees a separate image covering a wide area on each side of the head.