REFRACTIVE VISION DEFECTS
3 different refractive vision defects are existing: Myopia, Hyperopia and Astigmatism.
Myopia: Also called "nearsightedness" is a defect of vision in which far objects appearing blurred but close objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye's lens too strong. Miyopia can be corrected by wearing glasses/contacts that contain concave lenses.
Hyperopia: Also called "farsightedness" is a defect of vision where closer vision is difficult but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hyperopia can be corrected by wearing glasses/contacts that contain convex lenses.
Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.