Have you ever asked yourself why 20/20 is the standard for ''perfect'' eyesight and what it actually stands for? The phrase 20/20 eyesight expresses a normal level of clarity of eyesight (visual acuity) determined from 20 feet away from the object. That is to say that an individual with 20/20 eyesight will be able to see an object clearly from 20 feet away that most individuals should be able to see from that distance.
For those who don't have 20/20 vision, the number is determined based on the distance at which they are able to see clearly, in relation to what is normally expected. For example, 20/100 vision means that at 20 feet you can only see an object that the standard would see from 100 feet .
It's also possible to have better than 20/20 vision. For instance someone with 20/10 eyesight can see sharply at 20 feet what the average person can only see at 10 feet. Certain animals particularly birds of prey have more acute eyesight compared to what humans are capable of. For example, hawks have been known to have 20/2 eyesight, designed for locating prey from high in the air.
An average vision screening is done by using a vision chart most commonly the classic Snellen eye chart invented by Hermann Snellen, a Dutch eye doctor in the 1860's. While there are many variations, the chart generally has 11 rows with capital letters which get smaller in size as they move toward the bottom. The chart begins with the uppercase letter - ''E'' and subsequently adds more letters on the lines as they get smaller. During the vision screening, the optometrist will determine which is the smallest line of letters you can make out. Your score is determined since each line is assigned a rating, with the 20/20 row typically being assigned the eighth row. For young children, illiterate or disabled persons who can not read or vocalize letters, the ''Tumbling E'' chart is employed. Similar to the standard Snellen chart, the ''Tumbling E'' portrays only the capital E in different rotations. The person being tested uses their hand to point to the right, left, top or bottom according to the direction the E is pointing. Both charts needs to be placed 20 feet away from the patient's eyes.
Even though 20/20 visual acuity does show that the person's distance vision is good, this measure alone does not mean that a person has perfect vision. There are many other essential abilities needed that contribute to your overall vision such as peripheral vision, perception of depth, focus for near vision, color vision and eye coordination to name a few.
It's important to remember that even though a vision screening with an eye chart will often determine whether you require eyeglasses to correct for distance vision it doesn't give the eye doctor a complete understanding of your overall eye and vision health. It's recommended that you still go in for a yearly comprehensive eye exam which can identify any more serious conditions. Contact us now to book an eye exam in Mississauga, ON.