Anatomy
Refraction
Myopia
If the eyeball is elongated from front to rear, the lens does not need to be very curved to allow focusing of near objects, because the light rays will be able to travel further before they must converge on the retina.
However, in an elongated eye, even a fully flattened lens will cause light rays from distant objects to converge too soon, so that they will diverge again before they are received (focused on the retina). This causes blurred vision for distant objects, a condition known as "near-sightedness" or Myopia.
Myopia may also occur if the lens in incapable of flattening properly, or if the cornea is abnormally convex.
Read MoreHyperopia & Presbyopia
The opposite problem occurs when the eyeball is too shallow, the lens is incapable of thickening appropriately, or the cornea is less curved than normal.
In patients with these conditions, light rays from near objects do not converge in the eye, making vision of close objects blurred, while distant objects are seen clearly. This is "farsightedness" or Hyperopia (also called hypermetropia).
With advancing age, the lens loses its elasticity, making it more difficult for the lens to change shape and focus near images. This type of farsightedness is known as presbyopia. The term "presbys" means "old man."
Read MoreAstigmatism
Astigmatism is a structural defect of the cornea or crystalline lens that prevents light rays from converging at the same point on the retina, thus creating indistinct images.
For example, light rays from one part of the image may converge behind or in front of the retina, while light rays from another part of the image may converge on the retina. As a result, the image as a whole is distorted.
