In the absence of an eye-breaking defect, it is called "emetropia" and in this case the rays coming from a distant object parallel to the eye focus on the retina. It is called "ametropia" when the flaw is broken. Breaking defects are examined under 3 headings:
1. Myopia: Depending on the different causes, the focus is on the retinas. The most common reason is the normal length of time.
2. Hypermetropia: Hypermetropic rays focus on the back of retinas. The most important thing is that your eyes are shorter than normal.
3. Astigmatism: Rays from a point light source can not focus on retinas as a point, depending on whether the cornea (the transparent layer in the front of the eye) or the lens (the lens in the eye) are not the same in all respects.