The Eye and Vision

The eye is an optical system, comparable to a camera. The most important elements that refract light are the cornea and the natural eye lens. A normal-sighted eye (emmetropic) conveys the bundled light onto the point of most focused vision on the retina. This is required, in order to see focused images. In the most ideal case, images close and far are focused both without vision aids. In order to see objects close-up, the eye has to alter the focal point of its lens. This occurs through changing the curvature and is known as accommodation.

If the focal point isn't exactly on the retina, rather in front of or behind, objects are not seen focused. Imperfect vision is the result.