The sense of vision consist of three major components :

• The eyes that focus an image from  the outside world on the light sensitive retina
• The system of millions of nerves that caries the information deep into the brain
• The visual cortex that part the brain where it is all put together.Fig(1).

Ouroptical system has the fallowing special feathers most of which are not available on even the most expensive cameras:

• The eye can observe the events over a very large angle while looking intently at an object directly a head of it.
• Blinking provides the front lens (cornea) with a built – in lens cleaner and lubricator.
• A rapid automatic focusing system permits viewing objects  as close as 20 cm one second and distant objects the next.
• The eye can operate effectively over a range of light intensity of about 10 billion to one (1010: 1) – brilliant daylight to very dark night.
• The eye has an automatic aperture adjustment (the iris).
• The cornea has a built –in scratch remover ; even though it has no blood supply it is made of living cells and can repair local damage.
• The eye has a self regulating pressure system that maintains its internal pressure at about 200 mmHg and thus keeps the eye in shape.
• The eyes are mounted in a well- protected casing almost completely surrounding by bone ,and each eye rest on a cushion of fat that reduces sharp shocks.
• The image appear upside down on the light – sensitive retina at the back of the eye ball but the brain automatically corrects for this.
• The brain blend the images from the both eyes giving us good depth perception and true three dimensional viewing .
• The muscles of the eye (fig(2)) permit flexible movement up and down ,sideways and diagonally  .

Focusing element of the eye:

The eye has tow major focusing components:

• The cornea : which is the clear transparent bump on the front of the eye that does about two – thirds of the focusing Fig(3) eye anatomy .

• The lens :which does the fine focusing .Note : the cornea focusing by bending (refracting) the light rays.

Other component of the eye:

• The pupil: is the opening in the center of the iris where light enters the lens.It is appear black because essentially all of the light that enter it absorbed inside the eye. Under average light conditions, the opening is about 4 mm in diameter.

• The iris: it aids the eye by increasing or decreasing incident light on the retina until the retina has adopted to the new lighting conditions ; and under bright light conditions it plays an important role in reducing lens defect.

• The aqueous humor: it fills the space between the lens and the cornea .This fluid mostly water is continually being produced and the surplus escapes through the drain tube the canal of schlemm. The aqueous humor contains many of the components of blood and provides nutrients to the nonvascular zed cornea and lens.
•  The vitreous humor:  is a clear jelly –like substance that fills the large space between the lens and the retina .It helps keep the shape of the eye fixed and is essentially permanent . it is some times called the vitreous body.

• The sclera: is the tough ,white , light- tight covering over all of the eye except the cornea .The sclera is protected by transparent coating called the conjunctiva.