What is a visual acuity test?A visual acuity test is a key part of any eye examination. It records how clearly a person is able to see high contrast, black on white images of various sorts, called optotypes. Most tests comprise letters that are sized from large to small on a chart, computer screen or book.
Snellen letter chart was the first vision test available in 1862. Its letters were constructed in accordance with known scientific evidence about the eyes and vision. Since then many letter charts and other accurate optotypes used for vision testing all adhere to that science. Test charts that use silhouette images do not conform to the science and are less accurate.
A visual acuity test gives a score which is a measurement of how clearly you can see. It is only one part of a full professional eye examination.
What do the visual acuity test scores mean?Visual acuity is recorded in two main ways, one is in the form of a fraction and the other is shown as a decimal.
FractionThe fraction score is usually referred to as a Snellen notation, from the person who devised the first letter test and scoring system. The numerator (top number) is the distance from the person being tested in feet (usually 20 feet) or metres (usually 6 metres). The denominator (bottom number) is the distance at which a person with normal eyesight should be able to see that size letter or picture.
For example, 20/20 (or 6/6 in metres) vision is well known as normal vision. The top 20 is the testing distance and the bottom 20 is the distance where this size should be seen if you have normal vision.
If your vision is 20/40 (or 6/12) it means that a letter size that a person with normal vision can see when it is 40 feet (or 12 metres) away, you can only see it when it is much nearer, at 20 feet. This is worse than normal vision. The greater the bottom number, the worse the visual acuity score.
The lowest figure usually represented in this way is 20/200 or 6/60. If the vision is worse than this the testing distance (top number) is usually reduced by bringing the chart closer. For example if the 200 (or 60) size letter can only be seen at a distance of 13 feet (or 4 metres), the vision is recorded as 13/200 or 4/60.
DecimalThere are several decimal scores, but the commonest and the one used in iSight test is called LogMAR. It is an improved scoring system based on a logarithmic sizing that is more consistent than Snellen. The scores can be converted easily, but not the usual way with decimal and fractions.
For example, 20/20 or 6/6 vision is 0.00 in LogMAR and 20/40 or 6/12 is 0.30 in LogMAR. iSight test provides all three of these numbers for you.
In LogMAR scoring each size is 0.100 more than the previous size and each letter or picture has its own value. If there are four letters in a row, each one has a score of 0.025. This means that each letter or picture that is correctly identified is scored individually, so making the visual acuity test score recorded more accurate.
What is normal vision?Normal vision is generally taken as 20/20 in feet or 6/6 in metres or 0.00 in LogMAR. This is considered to be a good average that most people achieve on visual acuity tests.
Many people have better than average vision with scores of 20/16 or 6/4 being possible. In LogMAR scoring, vision better than 0.00 becomes a minus figure and one size better than 0.00 is -0.10.
You do not need “normal” vision to pass a driving test. In the US and UK the pass is 20/40 or 6/12 0r 0.300.
Children’s vision develops during the first years of life from quite poor vision at birth to normal adult levels by about two years of age. Testing children’s eyesight before this age is possible with specialist tests and expertise, but results are often more variable than with older children due to differences in visual development.
Not all children with normal eyesight achieve a normal visual acuity score when tested. There are several reasons for this, but the main one is that the test is subjective and requires the child to concentrate and try hard, sometimes for quite a few minutes during the test. Children notoriously find this a challenge.
It is important that the testing is made easy and age-appropriate. Testing should also be fun and quick to do, so no repetitive training, or using letters that mean little to a pre-school child.
The pictures in iSight test have been thoroughly researched so we know that children recognise them and they are accurate in measuring vision.
There has also been lots of research done to check what level of vision to expect in children with normal eyesight of different ages, given their concentration ability.
Most research on this subject has found that it is quite normal for a child under 6 years to get a visual acuity score of 0.10 (one size below standard), but there should not be a significant difference between the scores in each eye of more than 0.050.
When should I take my child for a professional eye test?If you have any concerns about the appearance of your child’s eyes or suspect a sight problem from their behaviour, always seek professional advice. It may be helpful if you have already tested your child with iSight test to provide this information to the eye care professional, but their comprehensive test results will be more conclusive.
If the interpretation results on iSight test advise you to seek professional testing and you are reasonably confident that the test was performed correctly.
If your child has a close family member (parent, grandparent, aunt, uncle, cousin) who developed a squint or needed glasses before the age of seven, or if your family has any hereditary childhood eye condition.
If your child has a congenital hearing loss, or other congenital condition, as these children are more likely to develop eye problems such as squint and refractive errors (needing glasses).
If your child was born significantly prematurely and or had a low birth weight (below 5lb or 2.270kg) as these children are also more susceptible to developing a squint.