In the normal eye, light bends through the cornea and lens to focus on the retina. With myopia (nearsightedness - place cursor over image) the eye is longer (or cornea is steeper) causing light to focus in front of the retina. The image is therefore out of focus on the retina and vision is blurred.

In the normal eye, light bends through the cornea and lens to focus on the retina. With hyperopia (farsightedness - place cursor over image) the eye is shorter (or cornea is flatter) causing the focal point of light to be behind the retina. The image is therefore out of focus on the retina and vision is blurred.

In the normal eye, light travelling through the cornea in both the horizontal plane and vertical plane is refracted (bent) the same, so that the image is properly focused on the retina in the back of the eye. Vision is clear. With astigmatism, (place cursor over image), an abnormal curvature of the cornea in one axis, the horizontal axis (blue) in this example, causes light travelling in this plane to focus in front of the retina. The light reaching the retina in this plane is out of focus and vision is unclear.

Normally, when close objects are viewed, such as a book, the lens of the eye (yellow arrow) will change shape (place cursor over image) to cause light to focus on the retina (blue arrow). Close vision is clear.

With age, a condition called presbyopia causes the lens of the eye (yellow arrow) to insufficiently change shape when viewing close objects. The light will not focus on the retina. Reading vision becomes difficult. Reading glasses can correct presbyopia by allowing light to focus properly on the retina.