Not seeing clearly – Refraction and/or Spectacle Errors
In order to see clearly, it is necessary that light rays reflected from an object coincide exactly on the retina, so that a sharp image of this object can be formed. This precise convergence of the light rays takes place because the incoming light is bent or ‘refracted’ by a system of lenses in the eye, the cornea and the eye lens. In a perfectly average eye, the distance becomes sharp. see without effort. If you want to look closer, internal eye muscles adjust the eye lens for this purpose, this is called accommodation.
Few eyes are perfect and that is why many people have refractive errors. With a refractive error, the eye itself cannot refract the incoming light well enough to focus the image of an object precisely and sharply on the retina. In these cases, properly fitted glasses or contact lenses almost always give the desired result: sharp vision at a distance and/or up close. Most refractive errors develop during childhood and can worsen afterwards, but sometimes also improve. We distinguish the following types of refractive errors:
Nearsightedness (myopia)
With myopia, the incoming light rays are refracted too strongly. They then do not come together on the retina, but in front of the retina. This occurs if the eye is too long (and therefore the retina is too far away from the cornea and lens), or the cornea is too convex (and therefore breaks too much). This no longer produces a sharp image of a distant object on the retina. However, one can see a nearby object better, hence the name: myopia.
Farsightedness (hyperopia or hyperopia)
If the eye is too short or the cornea is too flat, an image of a distant object takes place behind the retina. The incoming light is not refracted strongly enough. The image is not immediately sharp, but by (involuntarily) straining (accommodating) the eye muscles, the image can still be sharply depicted on the retina. For close viewing, even more accommodation must be made. The extra effort to focus for both distance and near can cause fatigue and headaches. This often increases during the day, especially with a lot of reading or computer work. Sometimes focusing is not good enough at all and glasses are needed to see clearly at a distance.
Astigmatism
Astigmatism occurs when the refraction of light in the eye is different in one direction than in the other. This also produces a blurry image. This is usually due to the fact that the cornea is not exactly spherical but somewhat oval-shaped (compare a football to a rugby ball). It is also possible that the lens of the eye takes a different shape. In some cases there is an abnormality in both the cornea and the lens of the eye. Astigmatism usually occurs along with nearsightedness or farsightedness.
Age-related farsightedness (presbyopia)
As we age, the ability of the internal eye muscles and the eye lens to focus (accommodate) at close range decreases. This phenomenon begins to occur approximately from the age of forty. We call this presbyopia. Most people who previously did not need glasses find that they have to hold text they want to read further away. However, this makes smaller letters less legible. Reading glasses can solve these problems. (Computer glasses are similar but have a slightly different working distance).
