The eye is a tiny organ that measures about an inch long and weighs about 1/4 of an ounce. It is part of a complex system that translates light into images. Light enters our eye through the cornea and into the pupil. The pupil is the black hole in the middle of the iris, the colored part of the eye. Behind the iris is the natural lens. It focuses the light onto the retina, the inside layer of the eye. The retina contains cells that are sensitive to light. The image is then converted into electrical impulses that are sent through the optic nerve at the back of the eye to the brain.
All of this happens continuously and instantly to give us clear vision from near to far distances. But, our eyes don't always work perfectly. Their exact size and shape affect how well they focus light. These differences can cause some of us to have refractive errors such as myopia, hyperopia or astigmatism. In addition, over time, we all lose our ability to focus on near objects. This is a refractive condition known as presbyopia.