A refractive error is a vision disorder where the shape of the eye does not allow light to focus perfectly on the retina, resulting in blurred or distorted vision. The types of refractive error are short-sightedness (myopia), long-sightedness (hyperopia) and astigmatism. Presbyopia is also a type of refractive error, however it is caused by ageing changes in the eye rather than shape irregularities at the front of the eye. For many patients, laser or refractive surgery options can remove their refractive error, eliminating the need for corrective glasses or contact lenses and providing patients with clear and comfortable vision. For more information about laser and refractive surgery options available at The Eye Health Centre, click here.
Short-sightedness (Myopia) means that long distance vision is blurry, while short, or close vision is still clear. It occurs when the eyeball is too long, or the front surface of the eye (cornea) is too curved. This causes light rays entering the eye to bend and focus images in front of the retina, rather than on the retina.
Long-sightedness affects close vision and causes problems with near focus. It occurs when the eyeball is too short, or the front surface of the eye (cornea) has too little curvature. This results in light rays entering the eye bending and focussing at a point behind the retina, rather than on the retina.
Astigmatism is a type of refractive error caused by an axial irregularity of the cornea, meaning vision can be blurred or distorted at both long and short distances. It can occur in conjunction with myopia or hyperopia, or as a stand-alone vision disorder.
Presbyopia is a common condition that makes vision difficult at a normal reading distance. It is not a disease, rather part of the ageing process, affecting those over the age of 40. Close tasks such as reading and sewing become difficult, particularly in poor light. For example, you may find that you are holding your newspaper further away from your eyes to make the print clearer. You may also have difficulty concentrating when reading or you may find periods of close work result in sore eyes, headaches or tiredness.
It is important to understand how your eyes change their focus for viewing close objects. Normally they are focused for distance vision. To focus on close objects, a special muscle in the eye changes the shape of the lens. This process is called accommodation. With age the human lens loses its flexibility and is less able to change its shape. This is a completely normal ageing change, just like stiffening joints or greying hair. The loss in lens flexibility is the reason that close focusing becomes more difficult.