How The Eye Focuses Moving from the outer portion of the eye to inner, light passes through several structures before falling on the retina where the brain may process the light as a visual image. First, light passes through the cornea, the clear portion of the outer eye that contact lenses are placed on. This area is sensitive to touch. It usually remains clear unless there is an infection with scarring, trauma or vision correction surgery with complications. The normal cornea contributes approximately 45 diopters of focusing power to the eye.
Diopters are a unit of measure describing the ability to bend light.
The other structure to contribute to focusing light is the lens. The lens lies just under the colored iris and forms the back of the anterior chamber and the front of the posterior chamber. The lens is visible through the opening in the iris called the pupil. The lens of a young person can provide a range from one to up to 20 diopters of focusing power. This ability is termed accommodation.