Pigment cells behind the rods and the cones prevent reflection of light within the eye. These impulses travel along the optic nerve to the brain. In response to light, the rods and cones in the retina produce electrical signals that trigger impulses in the nerve cells to which they connect. This enables us to judge distances and pick out detail, and improves the sensitivity of vision when light is poor. Binocular vision means that we can see through 180° without moving the head, with an overlap of 90° between the right and left visual fields. Since our eyes are placed on the front of the head we have binocular (three dimensional, or 3D) vision. Some creatures have an even higher resolution – the buzzard’s retina, for example, contains one million receptors/s/mm 2. We have 200,000 receptors/mm2, giving excellent resolution as long as vision during infancy has not been impeded in any way. The eye’s strength of accommodation is determined by the number of light receptors (rods and cones) present in the retina, and how closely they are packed together. The lens focuses an image upside down on the retina the conscious mind interprets the image and “sees” it in its true position.
The closest point at which the eye can focus varies with age – from about 7cm (3in) in infancy to only 40cm (16in) in old age. When focusing on near objects, the lens becomes rounder and thicker. When focusing over long distances (over 7m/20ft), the lens is at its flatest and thinnest. The lens then focuses the light on the retina by changing shape. Light enters the eye through the pupil, which dilates or constricts according to lighting conditions, lens adjustment and emotions. The human eye is more efficient at accommodation (ability to resolve fine detail) than the eyes of most mammals. Electrical signals from the retina transmit the image to the brain, which interprets it as upright. The image is focused further by the lens onto the retina, where it appears upside down. Light rays reflected from an object are partly focused by the cornea before entering the eye through the pupil. The pupils automatically dilate or contract to control the amount of light that falls on the retina. The eye contains 70 percent of the body’s sensors and can deal with one and a half million simultaneous messages. A network of nerve cells on the surface of the retina changes the light signals into electrical nerve impulses and then relays them to the brain via the optic nerve. Cones are concentrated in the parts of the retina called the fovea centralis (central depression) and the macula. The retina contains 137 million cells, of which 130 million are rod-shaped and used for black and white vision, sensing movement and seeing in poor light, while seven million are cone-shaped and used to sense colour and pattern in bright light. The eye is sensitive to light, which is focused through the cornea and the lens onto the retina at the back of the eye. The eye shows less growth than any other organ between birth and adulthood. The sensory cells of the eye are an extension of the brain, budding out from the brain during fetal development.