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The mechanics of your eye

The mechanics of your eye

The eye is made up of many individual parts, which work together to enable us to see. The Iris is the coloured part that has a central opening which is black and called the Pupil.

The Cornea is a clear curved structure which is both a protective coat and a lens and is in front of the iris. The Anterior Chamber is the space between the cornea and iris. The anterior chamber stays formed from fluid also called Aqueous Humor and provides oxygen and nutrients to keep the interior of the eye healthy.

The cornea is the most powerful lens in the eye's optical system. It is barely visible, because it is made up of a clear tissue. It’s curved shape means it acts like a lens, providing two thirds of the eye’s focusing power. The remaining power is supplied by the Lens, which sits inside the front of the eye behind the iris.

The Retina is similar to the film in a camera and lines the back of the eye. It consists of nervous tissue and photoreceptors which converts light rays into electrical signals and sends them to the brain through the optic nerve.

The iris and lens work together like a camera, allowing light rays from an image to pass through to the retina at the back of the eye, where the image is captured upside-down. The brain then turns the image right-side-up. The lens changes shape to adjust the fine focus, depending on the distance between the eye and the object.

Sometimes the ability to focus clearly is compromised. This can be the result of the eye being abnormally short or long, or if the curvature of the cornea is too steep or flat. Both of these conditions can cause the focus point to fall inside or outside of the retina. This is common in long and short sighted people and referred to as a Refractive error.


Normally, your eye can focus an image exactly on the retina:


Nearsightedness and farsightedness occur when the focusing is not perfect.


When nearsightedness (myopia) is present, a person is able to see near objects well and has difficulty seeing objects that are far away. Light rays become focused in front of the retina. This is caused by an eyeball that is too long, or a lens system that has too much power to focus. Nearsightedness is corrected with a concave lens. This lens causes the light to diverge slightly before it reaches the eye, as seen here:



When farsightedness (hyperopia) is present, a person is able to see distant objects well and has difficulty seeing objects that are near. Light rays become focused behind the retina. This is caused by an eyeball that is too short, or by a lens system that has too little focusing power. This is corrected with a convex lens, as seen here:



Astigmatism is an uneven curvature of the cornea and causes a distortion in vision. To correct this, a lens is shaped to correct the unevenness.


As we grow older, the lens becomes less elastic. It loses its ability to change shape. This is called presbyopia and is more noticeable when we try to see things that are close up, because the ciliary body must contract to make the lens thicker. The loss of elasticity prevents the lens from becoming thicker. As a result, we lose the ability to focus on close objects.

At first, people begin holding things farther away in order to see them in focus. This usually becomes noticeable when we reach our mid-forties. Eventually, the lens is unable to move and becomes more or less permanently focused at a fixed distance (which is different for each person).

To correct this, bifocals are required. Bifocals are a combination of a lower lens for close vision (reading) and an upper lens for distance vision.