Ever question why 20/20 is the benchmark for ''perfect'' eyesight and what it actually stands for? The phrase 20/20 vision expresses a normal level of clarity of eyesight (visual acuity) calculated from 20 feet away from the object. In other words someone with 20/20 vision can clearly see an object from 20 feet away that the majority of individuals are expected to be able to see from such a distance.
For those who cannot see at 20 feet away, the number is determined based on the distance at which they are able to see clearly, in comparison to the norm. As an example, if your acuity is 20/100 that means that at 20 feet you can only see an object that someone with normal vision can see at 100 feet .
You can also have vision that is better than the norm. For instance a person that has 20/10 vision can see clearly at 20 feet what the average person can see only at 10 feet. Certain animals have been known to have incredibly acute eyesight compared to man. A hawk for instance can have 20/2 eyesight, designed for locating prey from high in the air.
Most eye care professionals use some form of the Snellen eye chart, created by Hermann Snellen, a Dutch eye doctor in the 1860's, to perform an eye test. While today there are quite a few versions, the chart usually shows eleven rows with capital letters which get smaller in size as one looks downward. The chart begins with the uppercase letter – ''E'' with the addition of more letters as they get smaller. During the vision screening, the optometrist will determine which is the line with the smallest lettering you can read. Every line is given a rating, with the 20/20 row typically being ascribed forth from the bottom. For small children, illiterate or disabled persons who are not able to read or vocalize letters, the ''Tumbling E'' chart is used. Similar to the traditional Snellen chart, this version portrays only the uppercase letter E in different rotations. The patient uses their hand to mimic which direction the ''fingers'' of the E are facing: right, left up or down. Either chart needs to be positioned at a distance of 20 feet from the patient's eyes.
While 20/20 vision does mean that the person's sight for distances is normal, this measure alone does not suggest that a person has perfect eyesight. Complete vision involves a number of other necessary abilities such as side or peripheral vision, depth perception, focus for near vision, color vision and eye coordination amongst others.
While an eye exam using a Snellen chart can conclude whether you require eyeglasses to improve distance vision it will not provide the eye doctor a full perception of your overall eye and vision health. You should still go in for a yearly comprehensive eye exam which can identify potential conditions. Contact us today to book a San Jose, CA eye test.