There are four types of refractive error (problems with focusing). Some of these can be combined. The four types of refractive error are myopia or short sight; hyperopia (hypermetropia) or long sight; astigmatism and presbyopia. Astigmatism and presbyopia treatment can be combined with either myopic or hyperopic treatments.
In the perfect eye, distant objects are naturally in focus on the retina at the back of the eye and are seen clearly; this is called emmetropia. In youth, as the object comes closer, the eye is able to change its focus by a process called accommodation, which keeps the object in focus. This is sometimes called "Eumetropia".
Myopia or Short Sight
If an eye is short sighted, it is able to see things in focus at short distances. Objects in the distance are blurred because the focusing power of the eye is incorrect for the length of the eyeball. The eye is too long for the focusing power of the front of the eye. Whilst eyes that are short sighted can achieve clear vision with spectacles or contact lenses in many cases this can also be treated by a variety of laser and other surgical techniques. Up to the age of 40, Femto-LASIK is the preferred technique for myopia up to a maximum of 10 dioptres if the cornea is not too thin, but between 40 and 45 satisfaction rates fall, and it is generally not recommended after 45 years of age. In this age group, lens-based surgery can be considered.
Hyperopia or Long Sight
With a long-sighted eye, it is usually possible to see things in focus that are a long distance away but close work is either difficult and tiring or impossible. This condition exists because of a disparity between the focusing power of the front of the eye and the length of the eyeball. In this condition the eyeball is too short for the focusing power of the eye but to some extent the natural process of accommodation is able to compensate and many such patients are able to maintain good vision whilst young adults. However, the ability to accommodate decreases with age and many of these patients become spectacle dependent in mid-life. Whilst hyperopia can be treated with contact lenses or spectacles, surgical techniques now exist to treat many cases, and hypermetropic individuals tend to be the most grateful group after lens-based surgery. Femto-LASIK can correct hypermetropia up to about +3.5 dioptres.
Almost all eyes have a small amount of astigmatism. It is not a disease but an optical state of the eye where the front of the eye, instead of being equally curved in all directions (such as the side of an orange or a football) is more curved in one direction than the other (such as the side of a lemon or a rugby ball). If significant astigmatism exists, then objects will appear blurred at all distances unless the astigmatism is corrected by spectacles, contact lenses or surgery. Surgery for astigmatism is usually combined with surgery for either myopia or hyperopia.
It is well known that people who have enjoyed good vision for distance will require reading glasses sometime after the age of 45. This is due to presbyopia, which is a normal phenomenon. It is caused by the progressive loss of the ability of accommodate i.e. change the focus of the eye, and is a normal developmental phenomenon. Whilst presbyopia has been treated with spectacles or special contact lenses, we now have surgical techniques available which will treat presbyopia in certain selected cases.