The defects in eyes are caused by ageing and due to size changing of eyeball and due to the changing of curvature of eye lens.
i. Due to the excessive curvature of eye lens the focal length of the lens decreases and hence the image, of objects far from the eye, forms in front of retina rather than on the retina, so the defect is called near sightedness or Myopia because the person can only see nearby objects distinctly.
ii. The power of accommodation of eye decreases with time or ageing due to the weakness of ciliary muscles and due to their weakness the focal length of the eye lens cannot be changed properly, hence the defect is called Presbyopia.
