Basic Principles
When a person has difficulty seeing an object that is far away or in close proximity, the lack of function is related to the inability of the eyes to focus light rays on the retina. The retina is the portion of the eye that transmits images to the brain. Much like eyeglasses, contacts work to re-focus the light onto the retina, resulting in a clear image.
Modification for Condition
The way a contact focuses this light depends upon the type of vision problem. For those who are farsighted (can see objects that are far away, but not those up close), the eye does not focus the light enough before it reaches the retina. Contacts to correct farsightedness then cause light rays to converge more quickly before reaching the retina.
Conversely, nearsighted people have eyes that focus light too quickly in front of the eye. Therefore, contacts to treat nearsightedness make light rays diverge more until reaching the retina.
Fit Considerations
Contacts are essentially thin, see-through plastic discs. When a contact is applied, it's placed over the cornea. The moisture that naturally is present in the eye helps to bind the contact lens in place. Your eyelids and frequent blinking help to keep the contacts clean and free of dust and debris.
Contact lenses must be fitted, and this fit is determined by the shape of the wearer's eye. The inner portion is designed to provide a comfortable fit. The outer portion works to correct vision.
Considerations for Type of Lens
In order to be able to wear contact lenses as a daily replacement for eyeglasses, the wearer must wear the type of lens best for their eye. Many different types exist--such as hard, soft, gas-permeable or toric lenses that can be worn for a day, days or even weeks at a time.
These contacts work differently, depending upon the conditions associated with the eye. For example, those with astigmatism (trouble focusing to create a clear image) may wear toric contacts, which help to correct this problem. Those with severely dry eyes may be required to wear daily disposable lenses, as extended-wear options can be drying to the eye.
These conditions make finding the right type of contact to work for your eye as important as the prescription of the contact itself, because they affect a person's ability to wear contacts every day, for many hours at a time.
