This refers to an eye where incoming light from the distance focuses perfectly on the macula, the central part of the retina.
The best vision is achieved without lenses or very low-powered lenses. Patients with emmetropia possess the best vision in the far distance, such as when viewing television, reading a street sign when driving or watching a sporting competition.
In patients who have emmetropia, are beyond 50 years of age and are hence presbyopic, lack of accommodation may make it difficult to see objects nearer than the very far distance. They will need glasses for near tasks and possibly even for intermediate-range activities.
In an eye that is short-sighted, incoming light is focused in front of the retina.
To achieve the best distance vision, a spectacle lens is required to change the focus so it meets on the retina.
Short-sightedness presents typically in teenage years or the early 20s with an inability to see distance objects such as a classroom front board. Short-sightedness is worse in the dark so may also present itself in the first instance as difficulty driving at night.
The greater the short-sightedness, the closer to the face the position of natural focus. Very short-sighted individuals will hold reading material only centimetres away.
On the other hand, low levels of short-sightedness provide good natural vision at distances of one to two metres. Good vision at this range is very useful for tasks such as looking at a supermarket shelf labels, viewing a computer screen or reading a menu in a restaurant with moderate font without the need for spectacle lenses.
With increasing age, visual difficulties at that intermediate range become more common. For example, when sitting down and talking to another person, often done from one to two metres, the merit of good intermediate-range vision without the need for glasses becomes obvious. For most patients, the inconvenience of needing the occasional pair for glasses to achieve the very best distance vision is outweighed by the convenience of having good uncorrected intermediate-range vision.
In cases of long sight, when the eye is in the relaxed state the light focuses behind the retina.
Lenses introduced in front of the eye bring the focus forward onto the retina. Younger people are able to overcome this as they still have accommodation: the muscles of the eye contract, bringing the light focus forward. With increasing age, this ability to adjust the focus with muscle contraction is lost.
Long-sightedness typically manifests as the patient requiring reading or magnifying glasses at a relatively young age, much earlier than the typical age of 50 years. Spectacle lenses are then required for distance viewing five to 10 years after requiring them for near viewing. These individuals will have often had many years of spectacle independence well into their 30s and ultimately be fully dependent on glasses for near and distance viewing.
The distance and near-vision correction required by these individuals can be achieved by separate pairs of glasses: one for distance and one for near. More common is the use of multifocal lenses, which provide distance and near correction built into the one frame and avoid the inconvenience of changing between two pairs of glasses.