How does the human eye work?
Vision, a main human sense, is the source of brain feed with information, impressions, images and shapes. The eye, despite its very small size, consists of several parts, each of which has its own responsibility. The overall function of the eye resembles that of a camera.
More specifically, the sun rays enter the cornea and are concentrated in the crystalline lens passing through the pupil opening in order to focus and be captured as an image on the retina. This image then passes to the brain and it is decoded. Therefore, in order to properly and clearly convey an image of the outside world to the brain, a perfect "cooperation" is required among the cornea, the crystalline lens and the retina. However, the length of the eye can vary and can affect the focus of the image and its formation before or after the retina, thus causing the so-called refractive errors.
What are the refractive errors?
The refractive errors or ametropias of the eye are the myopia, the astigmatism, the hypermetropia and the presbyopia.
Myopia is a refractive error in which the image that the eye receives from the outside world is not focused, as it should be, on the retina, but in front of it. The symptoms of a person with myopia are the blurred vision when he looks away, while his vision remains clear when he looks closely.
Myopia is considered to be the most common refractive error and, under normal circumstances, occurs at school age either because the length of the eye is longer than the normal eye length, either because the curvature of the eye is longer than the normal one, or because of both of these factors. It is also known that myopia can be inherited, especially in cases of high degrees of myopia.
Myopia is treated and corrected either with eyeglasses in which negative lenses ("diverging lenses") are placed, either with contact lenses, or by refractive surgery with Excimer Laser.
Hypermetropia (or Hyperopia) is a refractive error in which the image that the eye receives is not focused, as it should be, on the retina but behind it. The symptoms of a person with hyperopia are blurred vision when looking closely, while at the same time there is a gradual reduction of long-distance vision.
Under normal circumstances, hyperopia occurs either because the length or curvature of the eye is less than normal, or because of both of these factors. Most people with hyperopia under the age of 35, have the ability to see clearly without requiring any correction of the problem. However, due to the reduction of the adaptive capacity of the eyes due to the passage of time (adaptive capacity becomes zero at the age of about 60 years old), the ability to see at close range is significantly reduced, while at the same time various symptoms appear such as fatigue when reading, headaches, weakness in concentration etc.
Hypermetropia is treated and corrected either with eyeglasses in which positive lenses ("converging lenses") are placed, either with contact lenses, or by refractive surgery with Excimer Laser.
Astigmatism is the refractive error in which the image received by the eye does not focus on a specific point on the retina but on two lines perpendicular to each other (the "Sturm cone").
The symptoms of a person with astigmatism depend on the degree of the astigmatism this person has. If, for example, the degree is too small, there are no vision problems because the adaptive capacity of the eyes eliminates the problem of the refractive error. However, as the degree increases, the vision becomes blurred, the images are not clearly visible while it also causes fatigue in the eyes, especially during reading or a computer use.
Usually, astigmatism occurs due to disorders in the curvature of the cornea and more rarely due to disorders in the curvature of the lens. In most cases, astigmatism does not change throughout a person's life and appears at birth. Therefore, over the years, the increase in the degree of astigmatism is not due to the refractive error but due to the reduction of the adaptive capacity of our eyes.
There are two types of astigmatism: normal astigmatism, which is easily corrected, and abnormal astigmatism (which could be due to keratoconus, trauma, or keratitis), which is more difficult to be corrected. Finally, there is a possibility that the astigmatism coexists with myopia or hyperopia or even it could be "mixed" (for example, hyperopia with myopic astigmatism).
Astigmatism is treated and corrected either with eyeglasses in which cylindrical or spherical lenses are placed (when myopia or hyperopia coexists), either with contact lenses (soft toric or semi-hard, depending on the case), or with refractive surgery with Excimer Laser.
Presbyopia is a normal refractive error and development of the eye system that occurs in all people over the age of 40 and is due to changes in the crystalline lens of the eye.
The crystalline lens is extremely elastic in order to contribute to the correct focus of images at all distances. However, as the years go by, this lens gradually becomes inelastic, resulting in the loss of its ability to adapt vision, especially at close distances.
The symptoms of presbyopia are eye fatigue and headaches after working at close distance, and this refractive error may coexist with myopia, hyperopia or astigmatism.
Finally, presbyopia is corrected either with eyeglasses (with monofocal, bifocal or multifocal lens), either with contact lenses (multifocal or simple in combination with the monovision method, in which one eye is used to see near and the other in long distance) , or by surgery (with Excimer Laser or with the use of multifocal intraocular lenses).