Have you ever wondered what exactly is vision impairment? Maybe you’d like to know the difference between being totally blind or legally blind. And what is 20-20 vision?
If you or someone you care about is diagnosed with blindness or vision impairment, all of these terms can be daunting. Here we explain some key terminology to help you through.
A vision impairment is technically defined as a limitation of one or more function of the eye, or visual system.
This means, a person with vision impairment, or low vision, does have some useful vision, however their vision loss is severe enough to affect their everyday life. This can include affecting a person’s ability to perform vocational, recreational and/or social tasks. Low vision can’t usually be corrected to normal vision with regular glasses.
A person who is considered totally blind has no measureable or useable vision, and no light perception.
Legally blind is a term used by the Australian Government to identify people who are eligible for special benefits and services. If you’re legally blind, it doesn’t always mean you are totally blind, it means that you can’t see things clearly. Vision is tested one eye at a time, using a standard letter chart to assess your visual acuity. The chart has a line of letters in different sizes and the person is asked to read them from top to bottom.
How is Vision Impairment measured?
Visual acuity is a measure of what the eye is able to see at a set distance. In metric terminology it’s 6 metres for distance vision and 33 centimetres for near vision. Normal vision acuity is classed as 6/6 in metres. The first number shows the distance of the person from the chart in metres, while the second number shows the measured result. 6/60 means that a person six metres from the chart is seeing what a person with ‘normal vision’ can see at sixty metres. Many people will have heard the term 20/20 which is the same concept, but the measurements are in feet. It means you can clearly see an object 20 feet away. If you’re legally blind, your vision is 20/200, so if an object is 200 feet away, you have to stand 20 feet from it in order to see it clearly. A person with normal vision can stand 200 feet away and see that object perfectly. While many people will have heard the term 20/20, in Australia the metric measurement, 6/6 is fast becoming the common terminology.
Visual field includes a person’s peripheral vision in the measurement of their vision. That is, what can be seen all around while looking straight ahead. Visual fields can be measured using standardised tests.
Levels of vision and eye conditions
A person who cannot see at six metres what a normally sighted person can see at 60 metres, is considered legally blind. A person who is legally blind is someone who has less than 6/60 vision in their better eye or has a field of vision restricted to 20 degrees in diameter or less, compared to the normal field of vision of 180 degrees, or a combination of both reduced visual acuity and field of vision. Some conditions, such as Retinitis Pigmentosa (RP) affect mainly the visual fields, while Stargardt Disease and Macular Degeneration (MD) affect mainly visual acuity or central vision. People with RP and MD develop a complete loss of all sight, most retain at least limited vision. Depending on the degree of this limitation, the person may have legal blindness, howver they’ll still have the capacity to get around and do things.
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.