What does Visual Acuity mean?
Visual Acuity refers to your ability to discern the shapes and details of the things you see. It can be a measurement of your central vision at a distance or a measurement of your near vision (needed for reading for example).
An eye chart usually called a Snellen chart is used to measure your visual acuity at a standardised distance. These wall charts have letters or symbols in different sizes arranged in rows and columns.
Viewed from 3 to 6 metres away, the eye chart helps determine how well you can see letters and shapes. During an eye test, you are usually asked to sit or stand a specific distance away from the chart and cover one eye. You will read out loud the letters/ symbols you see with your uncovered eye. You will repeat this process with your other eye. Typically, you will be asked to read smaller and smaller letters until you can no longer accurately distinguish letters on the chart.
The smallest letters /symbols that you can identify on the chart is then your distance visual acuity.
Your visual acuity is expressed as a fraction, such as 6/6. The bottom number is the distance at which a person with normal eyesight can read the same line. If your visual acuity is measured to be 6/6 this means that your ability to see an object clearly from 6 metres away is normal.
If you have 6/12 vision that means you can only see at 6 metres an object that people can normally see from 12 meters away. If your visual acuity is 6/36 you can only see at 6 metres what someone with normal vision is able to see at 36 metres.
Near vision is traditionally measured with a hand-held card like an eye chart on a wall. If your distance visual acuity is not 6/6 you may need corrective lenses in the form of glasses, contact lenses, or further investigation by an ophthalmologist (eye doctor) as you might also have an eye condition, such as an eye infection or injury, that needs to be treated. You and your optician/ optometrist will discuss your test results as well as any treatment or correction that might be necessary.
It is important to note that even if you have normal distance vision, your near vision will normally require additional focusing called “accommodation” as you get older.
After age 40 to 50, additional assistance in the form of reading glasses or bifocals is usually required for normal near vision.
What is your Visual Field?
The entire area of what you can see i.e., your central and your peripheral vision combined is called your visual field.
What does a Visual Field test involve?
A visual field test is done to determine where your blind spots are located. It measures two things:
1. How far up, down, left, and right the eye sees without moving.
2. How sensitive the vision is in different parts of the visual field.
The visual field test can help find early signs of diseases that damage vision gradually.
A visual field test can also help find out more about the part of the nervous system that allows us to see. The visual part of the nervous system includes the retina, the optic nerve, and the brain itself. Problems with any part of this system can change the visual field.
The methods for measuring a person’s visual field range from simple screening tests that can be performed in a few minutes during a regular eye test to more complicated and comprehensive computerised tests.
If you have abnormal results on a visual field test, you might be asked to undergo additional tests or you might be referred to an ophthalmologist (eye specialist) for further evaluation.