Terms often used when talking about different levels of sight loss:

Visual acuity is a term used to describe the sharpness of the images seen. It is generally measured by a Snellen Chart which has nine rows of letters in decreasing sizes. Perfect vision can be expressed as a group of numbers i.e. 20/20, which means an individual can see the same detail at 20 feet as a standard observer can.

Visual field refers to what can be seen centrally (straight-ahead) and peripherally (off to each side and up and down).

Blindness does not necessarily mean a person is totally without sight, although of course it can do. Some people may have no vision at all, others may have some perception of light only and others will have a little residual vision.

The term ‘registered blind‘ (see later) is used when the vision of a person is reduced so much it meets a laid down criterion and the person wishes for this to be officially recognised as a disability.

Partially sighted relates to those people who have substantial visual impairment that does not amount to blindness. There is also an opportunity to opt for this level of visual impairment to be registered.

Low vision refers to a severe visual impairment where people are unable to read print at a normal viewing distance even with the aid of spectacles or contact lenses.

Short-sightedness (Myopia) is where someone has difficulty getting a clear image of objects which are in the distance e.g. difficulty in reading notice boards, problems seeing traffic approaching etc.

Long-sightedness (Hypermetropia) is where someone has difficulty seeing things which are close by e.g. reading or sewing.

Visual processing disorders refers to problems regarding making sense of what is seen through the eyes, i.e. a difficulty in the brain interpreting what it is seeing.