UK Keratoconus Self-Help and Support Association
KC sufferers are likely to be very familiar with the "Snellen Chart". Despite there being better means to measure your acuity, the Snellen chart is still the most common tool used today.
The Snellen chart tests Distance Visual Acuity (distance vision) and is only one of the tests done to access eyesight. A chart is used made up of capital letters, numbers, symbols or pictures which are larger at the top and smaller at the bottom of the chart. Distance visual acuity is usually measured at 6 metres and so the chart is often viewed through a mirror (because few consulting rooms are 6m long!).
The top line of the chart is of a size that could be read at a distance of 60 metres by a person with normal distance vision. The second line is of a size that could normally be read from a distance or 36 metres and so on.
What do the numbers mean?
This question is often asked, and yet confusion still reigns.
In the UK, your visual acuity (VA) is often stated as a pair of numbers such as "6/6" or "6/15". The first number is usually a 6, which simply means that the distance from you to the eyechart is 6 metres. In the USA, the first number is usually 20, representing 20 feet - approximately the same distance. This is of course the "20/20" vision everybody talks about (but few actually understand!)
If you have a VA of "6/60", this simply means that you can read the "6/60" line on the Snellen chart. This indicates that you can read the topmost letter on the chart (the topmost letter is almost always the "6/60", or "20/200" line) when standing 6 metres away. The "60" in this case means that a "normal" person should be able to read that letter at 60 metres.
So, the on the snellen scale, a VA of "6/18" simply means that you can read letters at 6m that a normal person can read at 18 metres. The larger the second number, the worse your vision is.
It is worth noting that the "normal vision" line ("6/6") is not usually the last line on the chart. There are usually one or two further lines, designed to test the VA of people with better than "normal" vision (for example "6/5").
Can I drive?
Under UK law, you must be able to read a new standard (post-2001) number plate from 20 metres away. The distance increases slightly to 20.5 metres for the slightly larger pre-2001 number plates. This roughly corresponds to a VA of "6/12". The legal requirement is that you must wear any visual correction (glasses and/or contact lenses) needed to read the number plates at all times whilst driving.
Test your own vision
We have a pair of Snellen Charts for you on this web site. The online one is a "quick and dirty" chart simply to allow you to track changes to your vision. You should always sit the same distance from your monitor (for example one arm's length) to make valid comparisons.
The printable chart is more accurate. You should stand with your heels 3 metres from the chart in good light, and you can read your VA on the right (numbers on the left if you prefer being "20/20" than "6/6"). The red arrow highlights the "normal vision" line, and the green arrow highlights the approximate driving standard. Please note that this chart is only approximate, and if you are in any doubt as to your fitness to drive you should consult an optician.
Of course, as you are only standing 3m from the printable chart, the top line should really be called "3/30" and the normal vision line "3/3", but things are confusing enough, so the chart is labelled using the more common numbers, even though it is designed to be viewed from 3m.