The human eye is our most important sensory organ because we recognize more than 80% of all environmental stimuli through the eyes. The eye is a very complex organ and is similar in function to a camera.
Incoming light is depicted by an image-refracting system (cornea and lens) that creates a sharp image in the light-sensitive layer (retina). The light rays that strike there are converted into electrical stimuli and transmitted via the optic nerves into our brain, where they are processed accordingly.
The iris (pupil) regulates the amount of incoming light by changing its width. This is controlled by the muscles in the iris.
The point in the retina where the rays of light are focused is called the "yellow spot" (macula). Although this area represents only a very small part of the retina (about 2%), it is essential for our reading ability. The remaining approximately 98% of the retina form the so-called visual field and are responsible for the perception of movement and objects in the middle and outer periphery of our visual area.
Ideally, distant and near-vision images are sharply focused on the retina without any aids. The eye has to change the focal length of its lens for nearness. This is called accommodation. For objects in close vicinity, the lens bends more and more, so that the incoming light rays are broken more strongly. For distant objects, the lens flattens, so that the light rays are focused in each case exactly on the retina.
A sharp image on the retina of the eye is the prerequisite for good vision. In order for the light-sensitive layer of the eye to really create a sharp image, many conditions must be met; for example the length of the eye must be in the right proportion to its refractive power. Only then can pictures be seen sharply. This is certainly the case with the normal-sighted eye. Very often, however, a so-called defective vision occurs. It affects our vision and usually needs to be corrected. Moreover, a clouding of the cornea or lens can lead to a noticeable deterioration of the visual acuity and necessitate a medical intervention. Good vision is thus based on the depictimg of sharp images on the retina of the eye.