Even the news program 20/20 promises a clearer view of specific stories.
Measuring Vision
Human vision is complicated.
It is multifaceted, and eye care professionals need some way to quantify or measure vision for patients.
PhotoAlto/Eric Audras / Getty Images
Snellen System of Visual Acuity
To many peoples surprise, aneye examinationin its entirety is quite complicated.
The first and maybe the most important measurement is our central vision, or central visual acuity.
In the United States, we use the Snellen system of visual acuity.
The top number, 20, refers to a standard testing distance in feet.
One minute of arc is equal to 1/60th of a degree.
The entire letter makes up 5 minutes of arc.
The angle is five minutes of arc for the whole letter and one minute of arc for each bar.
So What Does 20/400 Mean?
All the lines below the E are indistinguishable.
In contrast, someone with 20/20 vision, can see all the way down to the 20/20 line.
Interestingly, simply putting up a mirror simulates the 20-foot-long testing distance quite well.
With the advent of computerized eye charts, the calibration is even easier.
Is 20/20 Really Normal?
We have to determine 20/20 visionto be the average normal vision.
However, as with most measurements in health care, there is a normal range of vision.
The resolution of our central vision is similar to a high-resolution computer monitor.
If the pixels are very fine and close together, the resolution of the monitor is better.
TV manufacturers actually studyvisual acuity.
European Society of Cataract and Refractive Surgeons.Dutch master Herman Snellen famous for visual acuity testing chart.
Bahkir FA.Decoding the Snellen’s Chart.Delhi J Opthamol.
2020;30(4):68-70. doi:10.7869/djo.552
Britannica.Snellen chart.
Smith MD, Zink M, Malik A.Tested perceptual difference between UHD-1/4K and UHD-2/8K.SMPTE Motion Imaging J.
2020;129(6):43-51. doi:10.5594/JMI.2020.2999946
