Sight Defects
There are sometimes problems with the eyes which result in the image on the retina being out of focus.
Short Sight (myopia)
People with this problem can see things close by quite clearly but everything else is out of fcus and blurry. This can be caused by a lens when is too rounded or an eyeball which is too long. The result is the image comes to a focus in front of the retina instead of on it (the diagram is exaggerated for clarity). This problem can be overcome by wearing corrective lenses. The lenses need to be concave (fatter at the edges than in the middle) which cause the light to diverge slightly before entering the eye.
Long Sight (hypermetropia)
People with this problem can see distant things clearly but near-by objects become blurred. This is caused by either a lens which is too flat or by an eyeball which is too short. It can be corrected by using convex lenses (lenses which are fatter in the middle than they are at the edges). These lenses bend the light inwards before it enters the eyeball.
