Vision and Visual Acuity

The science of Optometry is the measurement of a persons’ standard of vision and how to best improve it where necessary. These days, Optometry is so much more than that, including diagnosis and management of numerous eye conditions and other visual problems.

However, the most important reason for checking a persons’ eyes is measuring how well they can see, and this is a measure of their Visual Acuity.

Many years ago, a man named Herman Snellen decided to try and evaluate what is a “good” level of vision and how best measure and record such evaluations. He invented what we call Snellen notation, which grades the level of vision each eye achieves with reference to a standard eyes level of vision.

After much research, he decided that a normal eye would at best be able to discern a letter of a particular size at a particular distance. This letter was about 8.9mm in size and could be just seen at 20 feet. This letter was then classed as the 20feet letter, and if the test was conducted at 20 feet, then that “normal” level of vision was called 20/20. It is often misinterpreted as “perfect” vision, but it is merely a benchmark of a good standard of vision, and most people with healthy eyes can see better than 20/20. In Europe, including Britain, we tend to think of these measurements in metric terms, and so standard Snellen charts are used at 6 metres, and so 20/20 in our terms is known as 6/6.

So, looking at the Snellen chart to the right, if a person viewing the chart at 6 metres with their right eye (RE) could see the line marked 9, then that eye sees slightly less than 6/6. If the left eye (LE) can see the line marked 5, then that eye can see slightly more than 6/6 and is better in this case than the RE. An optometrist would then conduct a number of tests to calculate what lenses are needed to bring light properly into focus on a patient’s eyes, and thus improve that patients visual acuity to its highest possible standard in each eye. this measure of a persons Refractive Error would then give us the Lens Prescription which could be used in glasses to give that patient better vision.