the age the normal lens becomes hard and looses its flexibility making difficult to see the near objects. The person needs additional
plus or convex lens to see the near object along with his distance correction. This is bifocal lens. The condition normally develops at age of 40. The problem with bifocals
is that distant and near objects up to 18 to 20 inches is seen clearly but objects beyond 18inches
can't be seen clearly. For this progressive lenses are available which have power to cover all the distance.