Visual Acuity



WHAT IS VISUAL ACUITY?

Visual acuity is essentially the clarity and sharpness with which you see things, i.e. how well you can see. It is a measure of how well your eye and your brain can distinguish spatial resolution. It is one of the ways that visual function is measured. Other ways of testing visual function include visual fields, color vision and contrast sensitivity.

Visual acuity is sometimes also referred to as Snellen acuity. In 1862, a Dutch ophthalmologist, Dr Hermann Snellen, designed the Snellen chart (example right) as a way of testing acuity.

(Image adapted from the internet)


Most charts have a series of alphabets or numbers, with the largest at the top. As you read down the chart, the letters gradually become smaller. There are some variations to the Snellen chart. For instance, the Tumbling E chart is useful for those who are unable to read. Instead of alphabets or numbers, the Tumbling E chart has the capital letter 'E' facing in different directions. The person tested has to determine the direction the letter 'E' is facing on each line.

In the research setting, the LogMAR or ETDRS (named after the Early Treatment of Diabetic Retinopathy Study) chart has become the gold standard way of measuring acuity. Its advantage over the Snellen chart is that there are 5 letters on each line, thereby reducing any effect from crowding and allows easier calculation of a visual acuity score.

Test your visual acuity with your free Snellen chart


CONDITIONS FOR VISUAL ACUITY TESTING

To measure your visual acuity accurately, certain conditions have to be fulfilled and standardized: contrast, illumination and distance.

Contrast has to be maximized. The way most vision charts achieves this is to have black symbols on a white background. This is because it is easier to distinguish black from white, rather than black from a dark shade of gray. Having maximal contrast eliminates the issue of contrast sensitivity in acuity testing.

Illumination of the object that you are seeing must be standardized. The room or area where you are having your eyes tested must be lit brightly enough for you to be able to see clearly. There must be sufficient light and brightness for the cone photoreceptors at the macula to be able to function at its optimum. On the other hand, too much brightness may introduce glare and affect the test. It has been recommended that a luminance level for acuity testing be equal or higher than 85 candelas per square meter.

Distance must be standardized. For distance acuity, you will need to essentially be able to see at infinity, i.e. at an unlimited distance away. Conventionally, 20 feet or 6 meters is taken as infinity from an optical perspective. This is the accepted distance when measuring distance acuity. If you test your vision nearer than the stipulated 6 meters, the chart letters will appear larger and you will be able to read further down the chart (and that's cheating, by the way...).

For near acuity (reading acuity), the distance is usually set at 16 inches or 40 centimeters. However, this is somewhat arbitrary because different people prefer to read at different distances from their eyes. As you grow older, your ability to accommodate and focus at near objects gradually decreases. This is called presbyopia. This is part of the normal aging process, even if you had perfect eyesight before that. You will find that in order to read clearly, the reading material has to be placed further and further away. This is usually noticed from the age of 40 onwards.

If there is insufficient light en