Refraction is used to detect your decreased vision by giving a appropriate glasses.
Both Myopia (short-sightedness) , Hypermetropia (long-sightedness) And Astigmatism (Distorted vision) are common eye conditions that mean light does not focus on the retina of the eye.
Myopia is the term used to define short sightedness. Light from a distant object forms an image before it reaches the retina. This could be because the eye is too long, or the cornea or crystalline lens is too strong.
Hypermetropia means long sight and is where the image of a nearby object is formed behind the retina. This could be because the eye is too short, or the cornea or crystalline lens does not refract the light enough.
ASTIGMATISM (cylindrical error) occurs when the incoming light rays are unable to reach a common focus within the eye. Astigmatism distorts or blurs vision for both near and far objects. The cornea is the clear front window of the eye. A normal cornea is round and smooth, like a basketball. When you have astigmatism, the cornea curves more in one direction than in the other, like a football. It is possible to have astigmatism in combination with myopia or hyperopia.
As you age the crystalline lens within the eye becomes increasingly rigid causing loss of focus at near range. Presbyopia occurs in all people regardless of refractive error.