Aberration of the eye
The word aberration refers to any distortion of the image projected on the retina which is caused by eye defects. The most common eye defects include nearsightedness, farsightedness, astigmatism and presbyopia. These defects are generally caused by a wrong angle of light falling on the retina, which causes the resulting image formed on the retina to be unclear, blurred or distorted.
Nearsightedness (Myopia) is defined as poor vision in the distance. Nearsightedness occurs when the cornea is too curved and the eyeball is too long. The result is that light focuses incorrectly and causes far-away objects to appear blurry. Myopia can be corrected by contacts with MINUS dioptric value.
Conversely, farsightedness (Hyperopia) happens when the lens is not capable of flattening. In this case, the eyeball is too short, and the light rays focus behind the retina, rather than on it. This causes close-up images to appear blurry. Hyperopia is corrected by using corrective lenses with a PLUS dioptric value.
Astigmatism is caused by deviation from the regular corneal spherical shape. It is a refractive error, causing problems in how the eye focuses light. In this case, light rays focus on multiple parts of the eye: either in front of the retina, behind it, or both. This eye defect results in blurred vision and poor spatial perception (distorted image). Astigmatism can be corrected by toric contacts.
Presbyopia is similar to farsightedness. However, it is not a defect in true sense of the word, but rather a natural consequence of the aging human body when the eye loses its natural elasticity. People with presbyopia find focusing on close-up tasks (reading, writing or sewing, for example) strenuous and often need to hold their material at an arm's length to see clearly. Presbyopia can be corrected by multifocal contacts.