Myopia (Nearsightedness) occurs when your eye is too long in relation to the curvature of your cornea. This causes light rays entering the eye to focus in front of the retina, producing a blurred image.
Hyperopia (farsightedness) occurs when your eye is too short in relation to the curvature of your cornea. With hyperopia, distant objects are seen more clearly than near objects.
Astigmatism occurs when your cornea is shaped like an egg. With astigmatism, more than one focal point within the eye distorts what you see. As a result, you experience a tilting of images due to the unequal bending of light rays entering your eyes
more on Astigmatism Presbyopia:
Presbyopia is a normal part of aging that becomes more apparent as people approach their 40s and 50s. As the human body ages, the lens inside the eye grows like rings on a tree.
A cataract is a clouding of all or part of the normally clear natural lens. This can results in blurred or distorted vision. Cataracts are most often found in persons over age 55; however, cataracts occasionally are detected in patients of all ages.
In Keratoconus the corneal tissue is weakened and progressively thins and steepens which can distort vision even with glasses. Often specialty contact lenses are needed to see clearly and in severe cases corneal transplantation is required to obtain better vision.