"I have 20/20 vision!".
How many times have you heard people brag about their 'perfect vision' in that way??
It is great to have 20/20 vision, but what does that term actually mean?? Is it really perfect vision?
And what about blindness? How many times have you heard someone say "I am SO blind!!"....right before they get behind the wheel!! They are obviously not blind in any way shape or form!
20/20 Vision Defined
The origins of 20/20 vision are a fairly interesting topic. The original examining lanes for eye doctors were 20 feet long. When doctors tested the vision of their patients, the patient was 20 feet from the eye chart. If they could read the lowest line on the chart without glasses, they were listed as 20/20, meaning that they could see those letters at a normal distance. The top number is always 20, because the patient is always the top number, and was always 20 feet from the chart.
When a person's vision is blurred, they can only see letters that are larger. So when we talk about 20/30 vision, it means that the patient is sitting 20 feet away from the chart, but a person with 'normal' vision could stand 30 feet away from the chart! 20/400 (the 'big E' on the eye chart), means that the patient is 20 feet from the chart, and a person with good vision could see the same size letters at 400 feet!
Legal Blindness Defined
When eye doctors discuss blindness, we are always referring to 'best corrected vision', meaning how well the patient sees with their glasses or contact lenses ON! So, while people may describe themselves as blind, patients are not truly blind unless they meet the criteria for blindness even when wearing corrective lenses.
Is 20/20 "Perfect"??
While 20/20 is usually the goal for correcting vision, it is really NOT perfect vision. If all conditions are met, the eye actually has the ability to see about 20/10 or 20/8, meaning that the patient can see the letters at 20 feet, while a 'normal' person has to stand at 10 or 8 feet (respectively).
