Humans and most mammals have Complex Eyes which use a very sophisticated lens system to focus light onto receiving
nerve sensors at the back of the eye. This type of eye cannot sample the outside world at the same fast speed of
the insect's compound eye, and is a lot slower. However, the information gathered is highly detailed.
|On the right is a diagram of a human eye.
The Retina has two distinct types of light-sensitive nerve cells: rods and cones.
Rods detect monochrome (black-and-white) imagery and are especially well suited to low light levels. There are
no rods at the Fovea or blind spot, and more exist for peripheral vision than central vision. The Fovea region
is highly important in the human eye, as it contains more cones than in other parts of the eye. This allows much
greater detail in our central vision (see A in diagram) where light waves are gathered and focused onto the Fovea
through the centre of the pupil. Thus we receive a sharper picture when we look directly at an object.
Cones enable color vision but they need brighter light to function than rods. Cones come in three types: long-wavelength sensitive (red colour detection), medium-wavelength
sensitive (green colour detection), and short-wavelength
sensitive light (blue
Cone and rod cells connect to nerve fibres of the optic nerve through intermediary cells in the retina. When light
falls on the rods and cones, nerves send electrical impulses into the brain.
The astounding things about human eyes is that they
auto-focus and they also adjust their irises to control light input. They are self-cleaning, and because we have
two of them placed side-by side, they provide us with two separate views of the external world - an attribute which
the brain uses by combining the two images into a single stereoscopic 3D image. This aids us in determining depth
and the relative distances of objects in our field of view.
Our eyes have a blind spot. This is the area at the back of the eye which is occupied by the optical disc. the
point at which the nerve cells leave the eye to connect to the brain. There can be no rods or cones in this area
to sense light. The alternate 3D model rendered below may also help to illuminate the parts of the human eye.
Watch a 3d model rotation of human eyes in situ
and relative to the human brain here.