The visual disorders (called refractive errors) that require you to wear glasses or contact lenses to correct shortsightedness, long-sightedness or astigmatism are the most common vision problems worldwide. Refractive errors occur when the curve of the cornea is irregularly shaped (too steep or too flat) or too short or long.
When the cornea is of normal shape and curvature, it bends, or refracts, the light on to the retina with precision. However, when the curve of the cornea is irregularly shaped, the cornea bends light imperfectly on to the retina. This affects good vision.
The goal of glasses, contact lenses and refractive surgery is to correct or improve these errors by helping images to focus closer to wall or onto the retina.
The refractive process is similar to the way a camera takes a picture. The cornea and lens in your eye act in the same way as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image. The image that your retina sees then goes to your brain, which tells you what the image is.
Myopia or shortsightedness occurs when the eye is too long or the cornea is curved to steeply. Far away objects will appear blurry because they are focused in front of the retina. Myopia effects over 25% of all adults.
Hyperopia or long-sightedness of occurs when the eye is too short or the cornea is too flat. Distant objects are clear, and close-up objects appear blurry. With hyperopia, images focus on a point behind the retina.
Astigmatism occurs when the cornea is not round like a football but curved more like a rugby ball. The uneven curvature of the cornea blurs and distorts both distant and near objects. This causes light rays to have more than one focal point and focus on 2 separate areas of the retina, distorting the visual image. Two-thirds of people with myopia also have astigmatism.
When you are young, the lens in your eye is soft and flexible. Then the lens of the eye changes its shape easily, allowing you to focus on objects both close and far away. The refractive error known as presbyopia begins to occur in most people around the age of 40 when the lens of the eye begins to lose its elasticity.
It is a normal ageing process, that occurs in everybody, which makes it difficult to focus on near objects or more difficult to read at close range and is usually corrected with bifocals or reading glasses. You can have presbyopia in combination with myopia, hyperopia or astigmatism.
+ What is the best method of correcting refractive errors?
There is no best method of correcting a refractive error. The most appropriate correction for you will depend upon your eyes, your lifestyle and your age. Refractive errors can be corrected with glasses (single focus, bi-focal, tri-focal or variable focus), contact lenses (generally soft or gas-permeable) or refractive surgery.
The decision for which one of these to opt for will depend on the advice that you receive from your optometrist, your ophthalmologist and your family practitioner.