Understanding Your Prescription

Short sight, long sight and astigmatism

We see by forming an image on the retina – a carpet of light sensitive cells, each acting like a pixel on a computer screen, that lines the inside of the back of the eye. With ‘normal’ vision (emmetropia), images are brought to a clear focus on the retina by the cornea (the clear part of the front of the eye) and the natural lens (suspended within the eye just behind the pupil).

Spectacles and contact lenses either add or subtract focusing power to help form a clear image on the retina where the natural focusing power of the eye is incorrect. Defects in natural focusing power are called refractive errors. Refractive errors are measured in units of lens power (“dioptres” or D) and are represented for each eye in your spectacle or contact lens prescription by a number prefixed by a sign (e.g. +1.00D or –12.50D). The sign indicates whether the spectacle correction required is for long (+) or short (-) sight.

short_sightShort sight (myopia)

Short sight is caused when the eyeball is too long, or the cornea too steep, or a combination of the two. Light rays enter the eye and are brought to a focus in front of the retina. A short-sighted person with a relatively mild spectacle prescription of only -3.00D will have no difficulty focusing on small print, but they will start to lose focus on objects more than around 30cm away and their distance vision will be blurred. It is unlikely that this person could make out even the largest letter on an eye-test chart.

Someone with severe short sight of -10.00D, for example, will have a focal point of only 10cm – enough to read small print at close proximity without spectacles or contact lenses, but their distance vision would be extremely blurred.


ICL_long_sightLong sight (hypermetropia)

Long sight is caused when the eyeball is too short, or the cornea too flat, or a combination of the two. Light rays enter the eye and are brought to a focus behind the retina.Long-sighted people often have clear distance vision but have difficulty in focusing on close objects or when trying to read small print.

Younger people are able to increase focusing power, or accommodate, to see near objects clearly. This is due to the flexibility of the shape of the natural lens.

As we get older, the natural lens becomes less flexible and the ability to accommodate diminishes. This condition is known as presbyopia, and is characterised by the need to wear reading glasses in later life. Presbyopia is often mistaken for long sight.

It is very important to note that even those with ‘normal’ vision will still need reading glasses from their mid-40s on.

Astigmatism

Short sight myopia and long sight hypermetropia are often accompanied by an element of uneven focusing power, which is known as astigmatism.Imagine that the cornea is shaped more like a rugby ball than a football. The extent to which the difference in focusing power leads to a distortion of the image is the amount of astigmatism.

Astigmatism is represented in your spectacle prescription by a second number and an angle (e.g. –4.00D at 80°) indicating the focusing power and the angle of correction required to correct the astigmatism.

The final part of your spectacle prescription describes the difference between your prescriptions for distance and reading glasses (known as reading addition). Typically, this varies from +1.00D in your mid-40s to a maximum level of +3.00D by your late fifties.

Understanding your prescription

Laser eye surgery is not normally performed until the spectacle prescription has stabilised, typically by the late teens or early twenties for short-sighted patients (prescriptions with a minus value), and later for long-sighted patients (plus prescriptions) who are able to compensate by accommodating (focusing for near vision) when younger, and often only become spectacle-dependent in mid-life.