It is a poor indicator of a person’s ability to see colored objects, moving objects or depth perception- all qualities of one’s vision. This measurement is useful for a relative comparison of clarity of vision in a standard situation. Also the measurement is using high contract- black letters on white background. Why? Well, 20/20 vision is measured statically for one- in other words while the person is not moving. Vision is a term that is too general to accurately describe what the 20/20 measurement denotes. While we tend to say “20/20 vision” a more proper term is 20/20 visual acuity. Why do you keep saying 20/20 visual acuity? If this is the smallest letter you can read WITH corrective lenses in front of your eyes, you are legally blind. The single “E” at the top of the chart represents 20/200 vision. If you can read the 20/10 line- your visual acuity is two times better than someone who can only read the 20/20 line. The smaller letters in lines below the 20/20 line correspond to better than 20/20 visual acuity. The letters get larger as the lines go toward the top of the chart and each line corresponds to worse visual acuity. If you can identify the letters on this line, but none of the smaller letters on lower lines, you have 20/20 vision. At this distance, one of the smaller lines at the bottom of the eye chart has been declared the standard- to correspond with normal visual acuity. In the Snellen measurement system- the top number represents the distance between the person and the eye chart. Could he even have dreamed that 158 years later optometrists all over the US would be doing a happy dance that the year 2020 is going to be their year. Herman Snellen, developed the 20/20 measurement system for visual acuity in 1862.
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