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Most of us inherit the basic shape of our eyes which determines how we focus. As we reach adulthood, changes in the shape of the eye stabilize and there is typically only minor variation, if any, in visual acuity. Over half the population will require no external vision aid to achieve clear vision. The remainder, however, will seek some form of correction to see clearly.
Defocus errors produce distorted vision that can be corrected with glasses, contacts, and possibly nonsurgical corneal reshaping such as Orthokeratology. Through a specially shaped external lens in your glasses or contacts, defocus errors are corrected and functional vision is attained. Orthokeratology is nonsurgical corneal reshaping that involves intermittent wear of tightly fitted contact lenses which reshape the cornea for temporary periods. With Surgical Vision Correction, however, prescription lens implants change the way you focus from inside the eye. There are two types of lens procedures that require prescription lens implants: Phakic Lens Insertion, also commonly referred to as Implantable Contact Lenses, involves a supplemental or additional lens inside the eye while Refractive Lens Exchange involves a replacement lens for the natural crystalline lens inside the eye. Types of defocus errors these procedures can correct may include the following:
A nearsighted person can see close images well but has difficulty with distant images. This is because images are focused in front of, or short of, the retina (hence, the term "short-sightedness"), usually the result of an elongated eye. Prescription lens implants extend the angle of refraction to produce a clear image on the retina.
A farsighted person can see distant images well but has difficulty seeing near images. This is because images are focused beyond the retina, usually the result of a shortened eye. Prescription lens implants shorten the angle of refraction so that images focus best on the retina.
Unlike nearsightedness and farsightedness, astigmatism is not considered a distance error. An astigmatic person has blurred vision at any distance because the cornea or structures within the eye have an irregular shape that results in more than one focal point, producing multiple images within the eye. Special prescription lens implants, called Toric lenses create a single focal point and therefore, a single image. When astigmatism exists in conjunction with a distance error, both can be treated with the same lens.
In youth and early adulthood, our eyes have a remarkable ability to adjust strength for varying distances of vision. This is because the natural crystalline lens inside the eye is flexible. It's composed of a gel-like material, enclosed in a delicate membrane surrounded by ligaments attached to eye muscles. As the muscles contract or relax, the lens can become thinner or thicker for clear vision at varying distances. This process is called accommodation.
As we mature, however, the lens and its supporting structures become less elastic and we have increasing difficulty accommodating near images such as print. This normal phenomenon, called presbyopia, typically becomes noticeable in our mid-to late 40's. We then turn to progressive lenses, bifocal lenses or reading glasses to assist with focusing near images.
Surgical Vision Correction procedures that involve only the cornea of the eye, such as Laser Eye Surgery, do not alter presbyopia because the surgery does not involve the source of the change: the natural lens of the eye. A more comprehensive solution to resolve both distant and near vision may be desired. That exists in the form of Refractive Lens Exchange, using the ReSTOR lens implant.