The shape of your eye determines how well your vision can focus. Light rays enter the eye through the clear cornea, then through the pupil and the lens. In a normal eye the light rays are focused onto the retina, a light-sensitive tissue lining the back of the eye. Signals from the retina are sent through the optic nerve to the brain, where they are interpreted as the images we see. Refractive errors exist because the curvature of the eye is mismatched to the length of the eye and light rays cannot focus properly on the retina. It is technically very complicated to change the length of the eye (although it has been attempted) so refractive surgery procedures either change the shape of the front surface or add a permanent lens to improve the focus.

Myopia (Nearsightedness)
Myopia is the most common refractive error, affecting more than one in four people in North America. Myopic patients are nearsighted; they see close objects more clearly, but distant objects are blurry or even indistinguishable. Myopia occurs when the eye is too long or the cornea is too steep. As a result, light rays entering the eye fall in front of the retina instead of directly on it. The condition can be inherited and usually starts in childhood and stabilizes in the late teens or early adulthood. Traditionally, corrective lenses such as eyeglasses or contact lenses have been prescribed to refocus light entering the eye directly onto the retina.

In myopia, the eye is too long or the cornea is too steep.
Distant objects appear blurry because images focus in
front of the retina instead of on it.

Hyperopia (Farsightedness)
Hyperopic patients are farsighted; they can focus on more distant objects, but close-up objects appear blurry. Hyperopia occurs when the eye is shorter than normal or the cornea is too flat. As a result, light rays are not focused by the time they reach the retina. In young people, the natural lens can sometimes correct the focus in hyperopic eyes. With aging, the natural lens loses this ability and the vision blurs. This is why many hyperopic patients do not become aware of their condition until they enter their 20s or 30s.

In hyperopia, the eye is too short or the cornea is too flat.
Close objects appear blurry because images focus beyond
the retina.

Astigmatism
Astigmatism occurs when the curve of the cornea, and sometimes the lens, is uneven-steeper in one direction than the other-like a football or the back of a spoon. Almost everyone has some degree of astigmatism, but for some the uneven curve causes light rays to focus on many points in the eye and distorts both close and far vision.

An astigmatic eye has an uneven cornea. Images focus in
front of and beyond the retina, causing both close and
distant objects to appear blurry.

Presbyopia
Presbyopia is the age-related loss of close-up focusing ability. When we are young, our eye's lens is soft and flexible and can change shape easily, allowing the eye to focus on objects both close and far away. As people enter their 40s, the lens becomes less flexible and is unable to focus on close-up objects. Initially the blurring is worse in dim light, which is why many people first realize they have presbyopia when they have difficulty reading a menu. Later, the fine print in newspapers, magazines and phone books appears blurry. Near vision is most often corrected with reading glasses or bifocals, but refractive surgery can be used to create monovision in people who want correction for close-up vision. See the section on "Monovision" for more information.

With presbyopia, the eye's lens can no longer change
shape to focus on close objects. Images are not in focus
by the time they reach the retina.

Reading glasses or bifocals bend the light rays to move
the focal point onto the retina.