The inability of ciliary muscles to change the thickness of eye-lens as per requirement and to form the image of an object on retina leads to the defect of vision.
The three main common defects of vision are :
(i) Myopia or near-sightedness: It is the inability of eyes to see distant objects clearly. It arises when the eye lens does not become thin as per requirement and as such the image of the object is formed at a distance shorter than retina.
Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.

(ii) Hypermetropia or far-sightedness: It is the inability of eyes to see nearby objects clearly. It arises when the eye-lens do not become thick as per requirement and as such the image of the object is formed at a distance beyond than retina.
Hypermetropia can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.
(iii) Presbyopia: This defect of eye arises with aging due to the weakening of ciliary muscles and loss of elasticity of eye-lens. Due to this defect, a person finds it difficult to view nearby as well as distant objects clearly. This defect is corrected by using a bifocal in the spectacles of the person of such a person