Defective vision means you can't see clearly. In most cases, there is a purely optical problem, i.e. the visual defect can be corrected with glasses, contact lenses or with a surgical procedure.
The eye can be compared to a simple photo camera. The cornea corresponds to the rigid front lenses of the photo camera and is used to reduce the image of an object (2). The natural eye lens corresponds to the back lens of a photo camera and is used to focus on different distances. In between is the iris (iris of the rainbow), which, depending on the light intensity and viewing distance, regulates the incidence of light, like the aperture of a photo camera. The retina (3a/3b) in the eye is like the film of a photo camera; incoming light is transmitted to the brain in the form of stimuli via the visual path.
Light rays from afar are almost parallel and therefore do not have to be refracted as much as those from close up. The eye lens must therefore refract light rays to different degrees depending on the distance of an object. This focusing from far to near is called accommodation and is made possible by the fact that the lens is curved to different degrees. This allows us to see sharply in the distance and also in the vicinity. If, regardless of the distance of the object being viewed, the focus always falls precisely on the retina and therefore appears sharp, this is known as normal vision.
If there are deviations from this ideal state, this is referred to as defective vision
Depending on the type and severity of the visual defect, glasses or contact lenses can correct the light path so that it is focused on the retina and creates a sharp image.
Modern refractive surgery makes it possible to correct most visual defects. Depending on the type and severity of the visual defect, different techniques may be used.
