When light rays framework eye, breaking the. Mining means change of light direction. When the lightbrokenin the eye, it will say that the beams together. It is primarily the cornea and lens, in violation of the beams because of their structure and curved shape. The collector, then the beams in a certain angle. This means that the beams in a certain point back of the eye will all meet and then cross. To the point where all the beams meet, the picture is quite sharp.
This point where all the beams meet, must strike the retina, so that everyone can see things clearly. As the angle of refraction, as I said, is defined, this requires that the retina is the same number of millimeters behind the cornea, as radiant hub. This is the case in the "perfect end" but is very rarely the case in reality. In fact, the eye usually too far (myopia) or too short (hypermetropia), which will make that radiation focal point is either ahead
There are four standard refractive errors. Myopia and hypermetropia acting both on the surface of the eye length. Gammelmandssyn (presbyopia) is an age of increasing hypermetropia. However, this is not the eye length, but a rigid lens. Construction Error (astigmatism) are usually caused by the cornea is oval instead of round, and rays therefore not broken evenly over the place. Here in creating a slightly blurred image, which at near-and hypermetropia.
Each refractive error is described in the short-sightedness, hypermetropia, construction defects and gammelmandssyn.