Short-sightedness is a type of refractive error where good vision is given when looking at close objects but when the image becomes blurry when looking at distant objects. The eyes of people with normal vision manage to concentrate incoming light on the retina. This process is rendered possible by the so-called dioptric apparatus. The reason for myopia lies in a disproportion between the length of the eyeball and the refractive power of the eye. This disproportion causes light coming from distant objects to be focussed not on the retina but in front of it. For short-sighted people, the far point (the farthest point from the eye at which images are clear) does not lie in an infinite distance but rather close to the eye. Myopia can occur in two different forms. In the case of refractive myopia, the refractive power of the dioptric apparatus is too strong compared to the length of the eyeball. This condition can be caused by an opacification (clouding) of the lens (cataract), a weakness of the suspensory ligaments of the lens (zonular fibres) or by a deformation of the cornea. As regarding to axial myopia, the refractive power of the eye is normal while the optic axis is too long. In this case, hereditary factors can play an additional role.