The normal eye works on the same principle as a camera. The light rays or images come through the cornea, which is the clear curved membrane that covers the front of the eye. The light rays then pass through the lens, which is behind the pupil, and are then focused onto the retina. The retina, which is in the back of the eye, acts as the film of the camera and produces the image. The image is then transmitted to the brain to be developed and the picture is then "seen." If the light rays are not focused on the retina, then you are known to have a refractive error.

MYOPIA, or nearsightedness, is when the eye is too long and the cornea is too deep, causing the light rays to be focused somewhere in front of the retina. This causes blurred vision for distant objects. The degree of nearsightedness determines at what distance your vision is blurred.

HYPEROPIA, or farsightedness, is when the eye is too short and the cornea is too flat. This causes light rays to be focused somewhere behind the retina. Farsightedness causes blurred vision with near and distant objects.

ASTIGMATISM is an irregular cornea, similar to a football, shich causes multiple focal points on, before,and behind the retina. Astigmatism causes blurred vision with near and distant objects, and can occur alone or with nearsightedness or farsightedness.