Understanding How Your Eyes Function
The eye acts like a camera with two focusing components, the cornea and the natural crystalline lens. Clear image viewing is dependent on the light bending properties of these structures. The retina acts like the film of the camera, receiving the focused light rays.
When light is ideally focused, an eye may see 20/20. Conversely, when light rays are not optimally focused, images may appear blurry. This is called a "refractive error." The eye can have one or a combination of refractive errors, including myopia, hyperopia, astigmatism, and presbyopia.