A vision impairment is technically defined as a limitation of one or more function of the eye, or visual system. This can range from mild vision impairment to total blindness. It cannot be corrected to normal vision with prescriptive lenses or surgery.
A person with vision impairment may have some useful vision, referred to as functional or residual vision, however the vision loss is severe enough to affect the ability to perform vocational, recreational and/or social tasks.
Causes of vision impairment
Some people have congenital vision impairment (present from birth), whereas others have a degenerative eye condition or one acquired through trauma, eg. accident or brain injury.
How is vision impairment measured?
Vision impairment is determined through two aspects – visual acuity and visual field.
Some eye conditions, such as Retinitis Pigmentosa (RP) affect mainly the visual fields, while Stargardt’s Macular Dystrophy and Macular Degeneration (MD) affect mainly visual acuity or central vision.
Visual acuity is a measure of what the eye is able to see at a set distance. In metric terminology, the distance is 6 metres for distance vision and 33 centimetres for near vision.
Vision is tested, one eye at a time, using a standard letter chart. The chart has a line of letters in different sizes and the person is asked to read them from top to bottom.
Visual field includes a person’s peripheral (side) vision in the measurement of their vision. That is, what can be seen all around, while looking straight ahead.
Normal visual field is around 180 degrees across the horizontal. It is limited to around 50 degrees above and 70 degrees below, due to our facial structure.
Visual fields can be measured using standard computerised tests, which usually involve a person sitting with their head in a bowl-shaped machine and pressing a button when they see a light flash in their peripheral vision. The position of the lights are randomised.
At VisAbility, our Orthoptists use a Bjerrum field, which involves the client looking at a black screen and the examiner bringing a white dot on the end of a black stick from the edges of the visual field towards the centre, until it can be seen by the client.
What is 6/6 vision?
“Normal” vision is classed as 6/6 in metres. This is also known as 20/20 vision, which is the imperial equivalent.
The first number defines the distance of the person from the chart in metres. The second number shows the measured result.
A person with normal vision should be able to stand 6 metres away and see the chart clearly, although the lower lines should require some effort to identify.
6/60 means that a person six metres from the chart is seeing what a person with “normal vision” can see at 60 metres.
Levels of vision impairment
Vision impairment and blindness are very broad terms to describe less than 6/18 vision. There is a very wide spectrum, ranging from mild vision impairment to total blindness.
Depending on the degree of limitation, someone may have legal blindness, however, they still have the capacity to get around and do things.
If you, or someone you care about is diagnosed with blindness or vision impairment, some commonly-used terms can be very daunting and we have explained some of them below:
What is legally blind?
Legally blind is a term used by the Australian Government to identify people who are eligible for special benefits and services.
If you are legally blind, it simply means that you can’t see things clearly. You do not have to be totally blind to be legally blind.
A person who cannot see at six metres what a normally-sighted person can see at 60 metres or has a field of vision restricted to 20 degrees diameter or less (normal field of vision is 180 degrees) or a combination of both (reduced visual acuity and field of vision) is considered legally blind.
Legally blind prescription
Legal blindness is defined as someone with less than 6/60 vision.
A person who is considered totally blind has no measureable or useable vision and no light perception. It is estimated that less than 2% of people have total or partial blindness in one or both eyes*.
Find out more
Learn more about the most common eye conditions we provide support for. If you have concerns about your vision, our low vision assessment services can offer support and further advice. Or you may like to find out more about financial entitlements for people who are legally blind.