The eye is the human body’s camera in the way it focuses light to allow images to be sent to the brain such that we see. It has various tissues and structures which allow this to occur.
The cornea is the transparent curved tissue that covers the front part of the eye through which light first passes into the eye. The cornea helps focus the light as it enters the eye. Inside the eye is the iris, which is what gives people different coloured eyes such as blue or brown. The iris has a circular opening called the pupil which can change shape in response to lighting conditions to vary the amount of eye that enters the eye. From the pupil, the light passes through the lens which, when we are younger, is able to change shape to allow us to focus on things up close as well as at distance. Light then passes through the vitreous which is the clear jelly that fills the majority of the eye before it hits the retina. The retina is the thin tissue that lines the inside of the eye and acts like the film in a camera. It senses light and sends the light information through the optic nerve to the brain where we perceive an image.
Refractive Errors
In the normal eye, light passes into the eye and is focused sharply on the retina producing a clear image.
A person is said to have a refractive error if the light is not properly focused on the retina. There are 3 main kinds of refractive error:
Myopia (nearsightedness): a person can see things up close, but objects in the distance are blurry. This usually results because the eyeball is longer than average causing the light to be focused in front of the retina.
Hyperopia (farsightedness): a person has difficulty focusing on things up close, but may be able to see things in the distance clearly. People with a high degree of hyperopia have difficulty seeing at both distance and near. This usually results because the eyeball is shorter than average causing the light to be focused behind the retina.
Astigmatism: In a normal person, the cornea has the same curvature in all directions like a baseball, allowing light to be focused clearly on the retina. In someone with astigmatism, the cornea is curved more in the vertical or horizontal direction than in the other direction meaning it is shaped more like a football. This means the light does not focus to a single point causing images to be distorted. Astigmatism can occur along with both myopia and hyperopia.
Presbyopia: as we age, the lens in our eye gradually loses the ability to focus light for near and when most people enter their mid-40s they start noticing trouble seeing up close and may need to wear reading glasses. This naturally occurring process is called presbyopia.
All of the above conditions, for a person to see clearly requires wearing some type of corrective lens such as glasses or contacts lenses. There are surgical procedures that can be done such as laser refractive surgery (LASIK or PRK), refractive lens exchange (RLE) or phakic intraocular lens implants which can reduce or correct most of these conditions. Your doctor can advise you which procedures may be helpful for your condition.
