The Snellen chart is named after a Dutch ophthalmologist called, surprisingly, Herman Snellen (1834-1908) who worked all this out.
If you go to
http://www.keratoconus-group.org.uk/sne ... _3m_A4.pdf
download the chart and print it out, this will give you a 3m Snellen chart.
So what does all this mean?
Well, you have to have some method of standardising visual acuity and this is it. The UK has been metricated, as far as eyes are concerned, for a 100 years - so the chart works in centimetres and metres.
The average value of "Standard vision" was worked out using around 10.000 Swedish army recruits. What this boiled down to was that at 6m, letters (optotypes) on the 6/6 line could be read easily by the majority of the recruits. This is directly comparable to Imperial measurements as used by the USA ie 20/20 where the distance is expressed as 20 feet.
Technically the definition (according to Wikipedia) is:
Snellen defined “standard visionâ€Â
Snellen Test Charts
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