Ever ask why 20/20 is the standard for ''perfect'' vision and what it truly means? The term 20/20 eyesight describes a normal level of clarity of vision (visual acuity) determined from 20 feet away from the object. That is to say that an individual with 20/20 eyesight can see an object clearly at a distance of 20 feet which is considered normal to see from that distance.
For those who cannot see an object clearly at 20 feet away, the number is designated based on where they begin to see clearly in comparison to the norm. For example, if your acuity is 20/100 that means that at a distance of 20 feet you can only see an object that a person with normal vision can see at 100 feet away.
It's also possible to have vision that is better than the norm. For example someone with 20/10 eyesight can see clearly at 20 feet what the average person can see only at 10 feet. Members of the animal kingdom particularly birds of prey have more acute eyesight in comparison to the human species. A hawk for instance can have 20/2 vision, enabling them to identify prey from high in the air.
Most optometrists utilize some version of the Snellen eye chart, created by Hermann Snellen, a Dutch eye doctor in the 1860's, to perform a vision screening. While today there are many versions, the chart usually shows 11 lines with uppercase letters which get smaller in size as they move toward the bottom. The chart begins with the capital letter – ''E'' with letters being added subsequently as you look down the chart. During the vision screening, the optometrist will assess the line with the smallest lettering you can make out. Every line is given a rating, with the 20/20 line usually being assigned the eighth row. In instances where the patient isn't able to read, such as small children or handicapped persons, the ''Tumbling E'' chart is used. At the same scale as the traditional Snellen chart, the ''Tumbling E'' shows only the uppercase E in different directions. The patient uses their hand to point to the right, left, top or bottom based on the direction the E is facing. Either chart needs to be positioned 20 feet away from where the patient is viewing it.
Although 20/20 visual acuity does show that the person's distance vision is good, this test on its own doesn't indicate that a person has flawless eyesight. There are a number of other necessary abilities needed that contribute to your overall vision such as peripheral sight, perception of depth, color vision, near vision and focusing and eye coordination to name a few.
It's important to remember that even though a vision screening using an eye chart will establish if you require a visual aid to see far away it doesn't provide the optometrist a comprehensive perception of your overall eye and vision health. It's recommended that you still go in for an annual comprehensive eye exam which can diagnose any more serious conditions. Call us today to book a Scotts Valley, CA eye exam.