This section is to help you understand the medical or technical terms you will see on your prescription or may hear at the opticians. It's always good to have familiarity with such terms and be aware of what they mean.


This is when the front part of the eye (cornea) has the shape of a rugby ball instead of a regular circle. This makes it difficult to focus the light on the back of the eye, which causes your vision to be blurred.


A cataract is a clouding of the part of your eye called the lens. Your vision becomes blurred because the cataract is like frosted glass and it is difficult to see through.


This is when the optic nerve (which carries information from the retina to the brain) is damaged. Glaucoma can be caused when a blockage prevents fluid draining out of the eye and increases pressure in the eye, called intraocular pressure. Your eye needs a certain amount of pressure to keep the eyeball in shape and working properly. However, you can have problems if this pressure becomes too high.


This is known as long-sightedness, which makes it difficult to focus on objects that are close to you. Long-sightedness can be caused by the eyeball being too short, the cornea not being curved enough or the lens not being thick enough. In extreme cases, you may not be able to focus on objects at any distance.


This is known as short-sightedness, which makes it difficult to focus on objects in the distance. Short-sightedness is a consequence of the light not focusing at the back of the retina but just at the front. It is corrected with negative lenses that are thicker at the edge than in the middle, allowing the light to focus on the retina and produce a clear image.

Macular degeneration and age-related macular degeneration

Sometimes the delicate cells of the macula (an oval-shaped yellow spot near the centre of the retina) become damaged and stop working, and there are many different conditions which can cause this. If it happens later in life, it is called 'age-related macular degeneration' (AMD). There are two different types of AMD: dry AMD (caused by a build up of deposit) and wet AMD (caused by formation of abnormal blood vessels from underneath the macula).


This is when the eyes move uncontrollably. Your eyes will usually move from side to side, but they can sometimes swing up and down or even round in circles.


This is a muscle imbalance that could cause double vision as the eyes are not aligned with each other.


This is known as ‘old sight', and occurs when the eye can no longer focus on close objects. It usually happens around age 45, as the lens in the eye loses elasticity and becomes stiff with age. You need reading glasses if you have presbyopia .

Pupillary distance (PD)

This is the distance between the centre of your eyes. It is used to help make sure that each lens in your glasses is in the best position for your eye. Knowing the exact pupillary distance is more important the stronger your prescription is.

Retinal detachments

This develops when the retina starts detaching from the blood vessels that supply it. The retina cannot make a clear picture from the rays of light coming into your eye, and your vision becomes blurred and dim.


This is when your cornea (the clear front surface of the eye) is burnt by ultraviolet rays of light from the sun. This tends to happen when you are very high up in areas of snow, where the light reflects very strongly or if your eyes are insufficiently protected. Here are some of the symptoms of photokeratitis.

  • Your eye will be red.
  • Your eyelids will be swollen.
  • You will feel as though you have grit in your eye.

Ultraviolet (UV) light

UV light is invisible light from the part of the electromagnetic spectrum that is immediately beyond the violet end of the visible light spectrum. UV light is usually found in sunlight, and can both benefit and damage your health. UV light can cause problems for your eyes.

Ultraviolet B (UVB)

UVB is one of the three types of invisible light rays (along with ultraviolet A and ultraviolet C) which are given off by the sun.


Single-vision lenses

These are lenses that have the same optical power (the degree to which the lens converges or diverges the light) throughout. They can be used to correct short-sightedness, long-sightedness, astigmatism, or a combination of these.


These are lenses with two parts. The upper part tends to be used for seeing things at a distance and the lower part for seeing things close up, for example when you are reading.


These are lenses with more than two parts. The top part is for seeing things at a distance. This gradually increases in strength until the full reading prescription is reached near the bottom of the lens. You can use the part that gradually increases for seeing things int he middle distance, for example looking at a computer.

Find more info on eye conditions here.