Astigmatism
Astigmatism is a condition in which the cornea (or, much less often, the lens) is unevenly curved, causing light to be unevenly refracted. The result is that light may focus on the retina, but not on one focal point. This causes a blurry image, a condition known as astigmatism. A person can have astigmatism alone, or combined with either nearsightedness or farsightedness.

Cornea
The cornea is a clear, rounded surface in the front of the eye that covers the iris and pupil. It is one of the eye's refractive surfaces, that is, a surface that bends light entering the eye to focus the light on the retina.  The lens is the other refractive surface.

Epithelium
The epithelium is the thin, outer layer of the cornea.  It acts as a protective shield, stopping dirt, dust, bacteria, water and other foreign material from getting into the eye.

Farsightedness (see Hyperopia)
Hyperopia (also known as farsightedness)

Myopia is a condition in which the eyeball is too short or the cornea is not curved enough, causing light to focus behind the retina.  This causes distant objects to be sharp, close ones to be blurry. 

Iris
The iris of the eye is the colored tissue behind the cornea that gives the eye its color.  The iris controls the amount of light entering the eye by changing the size of the pupil.

LASIK surgery
LASIK (an acronym for laser in situ keratomileusis) is a form of refractive eye surgery that uses a laser to reshape the cornea.  This changes the way the cornea bends light and can correct a wide range of nearsightedness, farsightedness and astigmatism.

Lens
The lens is a somewhat flexible, clear, rounded surface behind the pupil of the eye. It and the cornea are the eye's refractive surfaces, that is, surfaces that bend light entering the eye to focus the light on the retina.  A small muscle around the lens helps it change shape, depending on whether we want to focus on a close or distant object. 

Myopia (also known as nearsightedness)
Myopia is a condition in which the eyeball is too long or the cornea is overly curved, causing light to focus in front of the retina.  This causes distant objects to be blurry, close ones to be sharp. 

Nearsightedness (see Myopia)

Presbyopia
Presbyopia is a condition in which we gradually lose the ability to focus on close objects. 

It's a natural result of aging that can start as early as forty, caused by the lens of the eye becoming less flexible, less able to change shape and alter the way it refracts light.

Pupil
The pupil is the black circular opening in the center of the eye through which light passes. Its size is automatically changed to control how much light enters the eye.

Retina
The retina is the inside back wall of the eye and consists of millions of visual cells.  The retina receives light, converts images from the eye's optical system into electrical impulses, and sends those impulses to the brain through the optic nerve.  When one has normal vision, light rays passing through the eye focus directly onto the retina, giving a clear, sharp image of close, middle distance, and distant objects.