What 6/6 actually means, and why 20/20 is the same number

A Snellen fraction is a comparison, not a score. Here is what the two numbers are, why the chart steps the way it does, and what it does not tell you.

Almost every patient reads their result as a mark out of ten. It is not one. It is a ratio, and both of its numbers are distances.

The top number is where you stood

6/6 means: at six metres, you read the line that a standard eye reads at six metres. 6/12 means you had to be at six metres to read what a standard eye reads at twelve. The American form, 20/20, is the same statement in feet.

So a “bigger” bottom number is worse, and 6/60 — the top line of the chart — means you read at six metres what an unaffected eye reads at sixty.

Why the lines step by 1.26

A modern logMAR chart steps each line by 0.1 log units — a ratio of 10 to the power 0.1, which is 1.2589. That is why 6/60 is exactly ten times 6/6: ten steps of 0.1 make one whole decade.

The step matters because it is even. On the old Snellen chart the gaps between lines were irregular, so improving by “one line” meant different things at different parts of the chart. On a logMAR chart every line is the same size of step, every line has five letters, and the result can be scored letter by letter.

What acuity does not measure

Acuity is a high-contrast measurement of the very centre of your vision. It says nothing about contrast sensitivity, about your visual field, about how you cope with glare at night, or about the health of the retina behind it all. People with 6/6 acuity are referred from this practice most weeks.