In order to understand nearsightedness, farsightedness, and astigmatism, it is necessary to examine the similarities between the human eye and a camera. Both the camera and the human eye have three essential features: 1. A light-sensitive substance – in the camera, this is the film; in the human eye, it is the retina. 2. An opening to allow light to enter – in the camera, this is the aperture; in the human eye, it is the pupil. 3. A lens system to focus the light rays onto the film or retina. It is the lens system that is responsible for producing clear images of near objects and far objects.
The lens system of the human eye actually has four components – the cornea, the aqueous humor, the crystalline lens, and the vitreous humor. All four of these components help to focus the light rays onto the retina. The light rays entering the eye must be bent inward so that they can form a clear image on the retina. The first component of the lens system that the incoming light rays pass through is the cornea. The cornea is the transparent structure located in front of the iris (the colored ring in the eye with the pupil in its center). The cornea actually causes the majority of the bending of the light rays.
The crystalline lens of the eye is located directly behind the iris. The aqueous humor is a watery fluid located between the cornea and the lens. The vitreous humor is a very viscous fluid located between the lens and the retina. All three of these components of the lens system contribute to the bending of the light rays, but together they do not cause as much bending as the cornea does. The contribution of the lens is extremely important because, while the amount of bending accomplished by the cornea, the aqueous humor, and the vitreous humor does not change, the amount of bending caused by the lens can be varied.
The crystalline lens of the eye has a biconvex shape – a disc which is thicker in the middle and thinner at the periphery. Expanding outward from the periphery of the lens are suspensory ligament fibers, which attach the lens to a ring-shaped muscle called the ciliary muscle. The ciliary muscle can affect the shape of the lens. When the ciliary muscle is relaxed, it forms a large ring. Under these conditions, the fibers of the suspensory ligament pull the edges of the lens outward. This causes the thickness in the middle part of the lens to be less than it could be. If the ciliary muscle were to contract, it would form a somewhat smaller ring. When this happens, the outward pull on the lens is less than before. As a result, the middle part of the lens becomes thicker. The more the ciliary muscle contracts, the thicker the middle of the lens becomes.
The ability of the eye to vary the thickness of the crystalline lens is essential in order for the eye to see clearly both near objects and far objects. In order to form a clear image on the retina, the light rays coming from a near object must be bent more than the light rays coming from a far object. The thicker middle of the lens, the more the lens bends the light rays. Therefore, whenever the eye focuses on a near object, the ciliary muscle contracts and forms a smaller ring in order to allow the middle of the lens to become thicker. But when the eye focuses on a far object, the ciliary muscle relaxes and forms a larger ring, then causing the suspensory ligament fibers to pull the edges of the lens outward. As a result, the middle of the lens is less thick.
Nearsightedness (myopia) can be caused by either an eyeball that is too long or a lens that is too thick. In either of these conditions, the light rays coming in from a near object can still form a clear image on the retina. The light rays from a far object could form a clear image at some point in front of the retina. But by the time those light rays actually reach the retina, the image is blurred. In order to correct nearsightedness, a biconcave lens is placed in front of the eye. A biconcave lens is a disc which is thinner in the middle and thicker at the periphery. Such a lens bends the incoming light rays outward before they reach the components of the lens system of the eye. After being bent inward by the lens system of the eye, the light rays will form a clear image on the retina.
Farsightedness (hyperopia) can be caused by either an eyeball that is too short or a lens that is not thick enough. In either of these conditions, the light rays coming in from a far object can still form a clear image on the retina. The light rays from a near object would form a clear image behind the retina. But, of course, they never reach that point; the image is still blurry when it reaches the retina. In order to correct farsightedness, a biconvex lens is placed in front of the eye. This lens bends the incoming light rays inward before they reach the components of the lens system of the eye. After the lens system of the eye bends them inward still more, the light rays form a clear image on the retina.
An astigmatism is a defect in the shape of either the cornea or the crystalline lens of the eye. The cornea and the lens should have exactly the same curvature in all directions. If there is an astigmatism, the curvature is not the same in all directions. To understand the difference, visualize the surface of a beach ball. It has the same curvature in all directions. Now visualize the surface of a football. It does not have the same curvature in all directions. When an astigmatism is present, the individual incoming light rays are bent by different amounts. Therefore there is no point where all light rays come into focus to form a clear image on the retina. In order to correct an astigmatism, a lens is designed to have an astigmatism which is exactly the opposite of the astigmatism of the eye. When such a lens is placed in front of the eye, it counteracts the astigmatism of the eye.
Presbyopia is a loss of near vision which occurs as people age. It is the result of loss of elasticity of the crystalline lens of the eye. Elasticity is the ability of a structure to spontaneously resume its original shape after it has been stretched out of shape. Elasticity is what allows the lens to thicken in the middle when it is no longer being rolled out at the periphery. As the lens ages, its ability to thicken decreases. Since near vision requires a thicker lens, near vision is lost.