Visual defects can be of a different nature. There are several main visual defects:
– myopia or myopia;
– farsightedness or hyperopia;
– color blindness or color blindness;
– color agnosia.
Visual defects – myopia
Nearsightedness can occur for two reasons. The first reason is the lengthening of the eye while maintaining the correct refraction. The second reason is the excessively powerful optical refraction of the eye, which is more than sixty diopters, with an eye length within the normal range. For both the first and second reasons, the image obtained by the eye is not able to focus on the retina, but is located inside the eyeball. Only a focused image of objects that are at a short distance from the eye penetrates the retina.
To correct myopia, patients are prescribed wearing special glasses, which help to build a clear image of an object remote from the organs of vision, which a person can look at without stress. In order for a nearsighted person to see clearly, minus lenses are used, which bring the image of a distant object closer.
Visual defects – hyperopia
Farsightedness occurs due to too weak optical refraction of the organs of vision while maintaining the normal length of the eyeball. In addition, among the causes of hyperopia, a shortening of the length of the eyeball can be observed, provided that the refractive optical power is preserved.
Due to the fact that the farsighted eye has a rather weak ability to refract, that is, is unable to create focus on the retina, the tension of the eye muscles increases. This tension changes the curvature of the eye lens, and the eye in such a situation must adapt to the prevailing conditions. But this, as a rule, is not enough for normal focusing of the image. When looking at objects near the eyes, the muscles tense up even more. In other words, the closer an object is to the eyes, the farther away its image appears on the retina.
Hyperopia can be corrected with a pair of glasses with plus lenses or with contact lenses that help in imaging.
The eyes of a newborn baby are slightly squeezed horizontally, which is why all small children have a slight farsightedness. Vision is normalized during the development of the eyeball.
If the degree of farsightedness is small, then vision both near and far may be normal. But people complain of eye strain and headaches. Moderate farsightedness is manifested by poor near vision.
Visual defects – astigmatism
The cause of the development of astigmatism is the malformed cornea of the eyeball. In addition, the development of astigmatism is adversely affected by the displacement of the eye lens in relation to the refractive axis. Both the first and second reasons entail differences in the distances required to focus the image for different sections of the eye.
With the development of astigmatism in one eye, a combination of farsightedness, myopia, and normal vision can be observed . For example, it can happen that for the cross-section of the eye the distance required for focusing is sufficient, but for the longitudinal axis it is insufficient. In such a situation, the vision of this eye will be transversely myopic. That is, a person will not be able to normally distinguish between horizontal lines, but vertical lines will see normally. At a close distance from the eyes, on the contrary, the horizontal lines will be indistinct, and the vertical ones will be clearly distinguishable. This is due to the accommodation of vision.