Myopia (Nearsightedness)
Definition
Myopia, also known as nearsightedness, is inherited and is often discovered in childhood. Myopia is a refractive error, which results from a disorder rather than from disease. A refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image. As light enters the eye, the visual image comes to a focus in front of the retina, resulting in a defective, blurred or distorted view of distant objects.
Myopia often progesses throughout the teenage years, when the body is growing rapidly. People with high myopia have a higher risk of detached retina, which can be repaired with surgery, and glaucoma.
 
Hyperopia (Farsightedness)
Definition
Hyperopia, also known as farsightedness, is usually inherited. Children are often hyperopic which may lessen as an adult. Hyperopia is a refractive error, which results from a disorder rather than from disease. A refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image. As light enters the eye, the visual image focuses behind the retina resulting in a defective, blurred or distorted view of both close and distant objects.

Astigmatism
Definition
Astigmatism usually occurs when the front surface of the eye, the cornea, has an irregular curvature. This irregular shape results in two focal points of light, instead of one. The simple act of reading often creates headaches and eyestrain, since the image is never completely clear. The individual may have to re-read the word several times. Astigmatism often occurs with nearsightedness and farsightedness. Astigmatism is not a disease nor does it mean that you have "bad eyes." It simply means that you have a variation or disturbance in the shape of your cornea.

Presbyopia
Definition
Presbyopia is a condition in which the focusing ability of a person's eyes has decreased to the point where vision at his reading distance becomes blurred and difficult. The focusing lens becomes unable to change shape and focus on close objects. This results in blurred vision at a reading distance, as well as eyestrain. Presbyopia most often develops in people in their forties.

Amblyopia
Definition
Amblyopia, sometimes called a "lazy eye," occurs when one eye does not develop normal sight during early childhood. When one eye develops good vision but the other does not, the eye with decreased vision is called amblyopic. Usually, only one eye is affected by amblyopia. This common condition should be corrected during infancy or early childhood to obtain 3-dimensional vision and prevent permanent vision loss.
Amblyopia has three major causes: strabismus, unequal focus, and cloudiness in the eye.
Strabismus (misaligned eyes) is the most common form of amblyopia. The brain "turns off" the image from the crossed eye to avoid double vision and the child uses only the better eye.
Unequal Focus (refractive error) is an eye condition that can be corrected by wearing glasses. Amblyopia occurs when one eye is out of focus because it is more nearsighted, farsighted or astigmatic than the other, all conditions that can be corrected with glasses. The brain "turns off" the image from the unfocused eye, although both eyes can look normal to the observer. This is the most difficult type of amblyopia to detect and requires careful vision measurements by an eye doctor.
Cloudiness in Normally Clear Eye Tissue may lead to amblyopia. Any disorder that prevents a clear image from being focused inside the eye, for example, a cataract or cloudy lens inside the eye or a cloudy cornea at the front of the eye, can lead to the development of amblyopia in a child. This is often the most severe form of amblyopia.
It is not easy to recognize amblyopia. A child may not be aware of having one strong eye and one weak eye. Unless the child has a misaligned eye, or other obvious abnormality, there is often no way for parents to tell that something is wrong. However, it is difficult to measure vision in young children. Your optometrist is trained to estimate visual acuity in an infant by watching how well a baby follows an object with one eye when the other eye is covered. He or she will also carefully examine the interior of the eye to see if other eye disorders such a tumor or an inflammatory process inside the eye may be causing decreased vision.