Difference between Myopia and Presbyopia
Myopia
1) The defect in human eye in which one can see nearby objects clearly, but cannot see distinct object clearly is known as myopia or near sightedness.
2) In Myopia, ciliary muscles expands the lens, but doesn’t relaxed, so that the focal length of eye lens decreases, eyeball gets elongated.
3) Image of object forms before retina due to convergence of rays before retina since the focal length is decreased.
4) This makes person able to see nearby objects clearly but can’t make him/her able to see distinct objects.
5) To correct this defect, an ophthalmic doctor prescribes the concave lens of suitable power (or focal length) to use in spectacles.
6) Hence the power of lens for the person with myopia is negative.
Presbyopia
1.) The defect of human eye which is appears due to old age is known as presbyopia.
2.) The power of accommodation of eyes decreases due aging so the person finds difficulties in seeing nearby objects as well as distinct objects.
3.) Such people require bi-focal lenses.
4.) A bi-focal lens is combination of both concave and convex lenses.
5.) The upper portion consists of a concave lens and lower part is convex lens, so that both the difficulties in vision of nearby as well as distinct object can be easily overcome.
6.) Hence the power of lens of person with presbyopia is positive as well as negative.