|Normal Eye - When we view objects the normal eye receives information from light rays focusing at a distance. The cornea, at the front of the eye, bends those light ways to the crystalline lens, which further bends them until the image of the object is sharply focused on the curved retina at the back of the eye.|
Eye conditions that result in less than perfect vision are caused by unusual curvatures in the shape of the eyeball or age-related problems.
Farsighted Eye - Also called Hyperopia, sees distance better than near, results from light coming into focus behind the retina. To correct this problem a converging lens (one with + sphere power) is used to bend the light inward.
Nearsighted Eye - Also called Myopia, sees near better than distance, results from light coming into focus before the retina. In order to correct this problem a diverging lens (one with - sphere power) is used to bend the light outward.
Presbyopic Eye - As people age their eye loses the ability to accommodate (the natural capability to produce clear images on the retina by the stretching or relaxing of the crystalline lens) because the muscles around the eye becomes more rigid. This results in difficulty focusing on near objects, causing a condition known as Presbyopia.
This condition requires multiple vision corrections, all of which should be accomplished with a multi-focal lens.
Astigmatism - Astigmatism results from a cornea that is not spherical causing objects to appear out of focus in an otherwise normal eye.
A correction for astigmatism can be put into any type lens (single vision or multi-focal) and is found in the cylinder power of the lens prescription. Two different power curves are ground at right angles in the back of the lens to refocus the two focal points of the retina.