Whether it’s from your family doctor, eye doctor, or a driving test – you often hear about 20/20 vision or “normal vision.” But what does 20/20 vision really mean? Here we look at the levels of vision and what vision testing determines, including what is meant by 20/20 vision.
Vision testing is measured with a Snellen chart. The doctors use the eye chart to measure how well you see according to distance compared to other people. The chart is set up to display letters of different sizes, with the large text on the top moving progressively to the smallest text on the bottom.
When you have normal 20/20 vision it means you can read the line of letters at 20 feet away that are located on the fourth last line on the chart. With 20/40 vision you would only see the larger lettering at 20 feet, and so forth. For those with legal blindness they would have a vision level of 20/200 and would only be able to read the big E at the top of the chart. You can also have better than normal vision, known as 20/15, 20/10 or 20/5. These are indicated as the last three lines on the chart.
If your test determines that you require eyeglasses, your prescription will be written in a grid using a combination of abbreviations, terms and numbers. The abbreviations displayed on the left side indicates which eye has the error:
- D indicates the left eye (oculus dexter)
- S indicates the right eye (oculus sinister)
- U is for both eyes (oculus uterque)
Along the top of the grid will have terms that indicate the type of error:
- SPH stands for Sphere. This indicates how strong your lenses will need to be to correct your vision.
- CYL stands for Cylinder. This indicates an astigmatism and the strength needed to fix it.
- AXIS is used to describe the direction and degree of your astigmatism.
- ADD stands for Added Magnifying Power in the lower part of a multi-focal lens. If you require bifocals you will have a number in this category. It is needed to correct a presbyopia which is an inability to focus on close objects.
Beside each abbreviation and below each term there will be a number. The numbers are referred to as a diopter and are measurements of the refractive error. These indicate how much correction you require. The further away from zero the more correction you need. A plus indicates nearsightedness or trouble seeing things up close, and a minus indicates farsightedness – a condition where you have problems seeing things far away.
If you’re not sure what your vision level is or are having problems seeing certain print or text, call your eye doctor today to book an appointment to get a vision test.