A. The cornea
The cornea is the front part of the eye. It is separated from the atmosphere by a film of tears, on which the contact lens rests. Its appearance can be compared to the lens of a camera, a spherical form that flattens towards the rims.
The form and the regularity of the surface of the cornea are important factors in the choice of the geometry of a contact lens (diameter, radius).
B. The retina
The retina is a light sensitive part inside the inner layer of the eye. It consists of different types of photoreceptor cells that receive light and transform it into image-forming signals. These signals are transmitted through the optic nerve to the brain. In this respect, the retina is comparable to the film in a camera.
C. The lens
The lens is, comparable to a contact lens, a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens has, activated by the eye-muscle, the ability to change its surface curvature. By altering the shape the focal distance of the eye is changed. It can thus focus on objects at various distances, allowing a sharp image of the object of interest to be formed on the retina. This adjustment of the lens is known as accommodation. It is similar to the focusing of a photographic camera via movement of its lenses. Unfortunately, the ability for accommodation decreases with age and results to the inability of the eye to focus on nearby objects. This phenomenon is called Presbyopia (see below).
The Visual Defects
Myopia (near- or short-sightedness)
Myopia is a frequently occurring visual defect caused by an eyeball that is too long. The beams of light are focused in the vitreous inside the eye rather than on the retina at the back of the eye. This produces a blurred image. As a treatment for this visual defect, divergent lenses (negative potency) are used.
Hyperopia (far- or long-sightedness
People suffering from hyperopia also perceive a blurred image.
Contrary to the myopia, where the focal point lies in front of the retina, in hyperopia it lies behind the retia. To treat this visual defect, convex lenses are used (positive potency) in order to focus the light closer than its normal range.
Contrary to people affected by myopia, hypotic persons can compensate the deficiency to a certain extend by accommodation. However, this leads to a constant fatigue of the eyes. Since with growing age the elasticity of the lens decreases, auto-correction becomes impossible and hypotia has to be corrected with vision aid.
Astigmatism is an optical defect whereby vision is blurred due to the inability of the optics of the eye to focus a point/object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. The intersecting axis of a cross may be perceived differently, one in, the other out of focus. Thus, the refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e., the eye has different focal points in different planes). To correct this visual defect, a lens is created that corrects according to the respective deviation of the meridians. This implies, that the contact lens must always remain in the same position on the eye. The lens is therefore stabilized either by the pressure of the eyelid on the exterior surface of the lens or by gravity (ballast).
The first symptoms of presbyopia are usually noticed between the ages of 40-45. With age, the eye exhibits a progressively diminished ability of accommodation (compare to lens/accommodation above). The focus on a near object takes place behind the retina, as is the case in hyperopia. In order to correct this problem, convex lenses are used (positive potency) for reading and other tasks demanding close vision. For the far vision, no correction is needed. A contact lens treating presbyopie features thus an area with positive potency and an area with a powerless correction for far vision. Near vision Far vision
The above visual defects can occur combined, with exception of myopia and hyperopia. It is possible to produce contact lenses that correct myopia (or hyperopia) as well as astigmatia and presbyopia.