A healthy eye enables clear vision of images at different distances, adapting to changes in light intensity.
The visual process is complex and can be divided into different stages. In the first step, the light enters the eye by travelling through all the transparent organs: the cornea, the aqueous humour, the lens and the vitreous humour. These structures are responsible for ensuring that the light rays entering the eye focus on a single point on the retina, the macula to be specific, to form a clear image. The image formed appears inverted.
The retina is responsible for transforming the light signals into nerve impulses. These pass through the optic nerve to reach the cerebral cortex and thereby form a single image thanks to both eyes. The cerebral cortex is where the impulses are interpreted and where the correct image is formed. To ensure they merge, it is very important to receive the two clear images from both eyes to achieve three-dimensional vision.
The structure responsible for regulating the amount of light is the pupil. With strong light intensity, it contracts (miosis); whilst with little light, it dilates (mydriasis).
For the visual process to work, the other visual conditions need to come into play effectively, such as accommodation of the lens (to focus on both near and distant objects), chromatic vision and adaptation to darkness (thanks to the cones and rods, situated on the retina), or binocular vision (when the two eyes work together to achieve a three-dimensional image).
It is important for a balance to be maintained between all these structures to achieve clear vision. This is referred to as emmetropia. When our vision becomes blurry it is because one of these systems is not functioning correctly and the terms refractive defect or ametropia come into play.