### Visual Acuity and Snellen Chart

In China, the Snellen chart used to measure visual acuity (VA) are full of tumbling E, having the opening pointing to all four directions, up, down, left and right. This is reasonable, as ordinary Chinese people do not know English decades ago. I believe this is better than the one used at US, because not all of Latin characters are equally recognizable. It is obvious that there's much more difference between an H and an O than an O and a D.

Not just the chart itself, the scales of VA are different. China uses decimal number instead of 20/20 fractional system. I tried to find the way to convert the VA scale used in China to the one in US like crazy. But the searching is so time consuming. Before searching on this, I always think the search engines are so convenient to find information like this. Maybe I didn't use the right keywords. Anyway, the following table of equivalent values in different VA scales is the complete table I find, which helps us to translate any kind of chart which we may encounter.

 Row metric English decimal Jaeger (Near vision) % visual efficiency % of loss Size of E Function 1 6/60 20/200 0.1 J8 20% 80% 87mm Much handicapped but still can get around on one's own, can can recognize people and objects 2 6/50 20/166 0.15 30% 70% 75mm 3 6/38 20/125 0.16 J7 40% 60% 65mm 4 6/30 20/100 0.2 J6 50% 50% 45mm Can do routine work and look at movies. Car license plates are blurred. Dangerous to drive 5 6/24 20/80 0.235 J5 60% 40% 6 6/20 20/66.6 0.3 J4 65% 35% 30mm Can enjoy TV, but outlines of faces and letters are not sharp. Can still drive a car but needs great caution 7 6/10 20/33.3 0.6 J3 90% 10% 15mm Can pass driving licence or seamen certificate test 8 6/6 20/20 1.0 J1 100% 0% 9.25mm Normal eyesight

Actually you don't need this table, converting the VA from 20/20 system to decimal system is easy.
20/20 should be treated as a fraction 1.0 and 20/200 = 0.1

Nevertheless, I still couldn't get all my doubts cleared. In China, the normal eye sight (standard acuity) is 1.5, not 1.0 as converted from 20/20. I don't know whether Chinese ophthalmologists have a higher standard of eye sight, or simply 1.5 scale is totally different from the decimal scale.

Posted by dangerous on January 17, 2008 at 06:25 PM PST #

Found the answer to my last doubt on http://en.wikipedia.org/wiki/Visual_acuity#.22Normal.22_vision

Basically for humans, the maximum acuity of a healthy, emmetropic eye (and even ametropic eyes with correctors) is approximately 20/16 to 20/12. Convert it to decimal scales, it is 1.32 to 1.67, roughly 1.5.

Posted by gc on January 18, 2008 at 04:00 AM PST #

I'm pretty sure we have a table for converting alot of acuities on our site. If you can't find one there send me an e-mail or info request and we will send you one.

Posted by Chris on February 12, 2008 at 01:34 AM PST #

Posted by gc on February 12, 2008 at 01:54 AM PST #

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