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20/20 (or 6/6) is a measure of how well you see. It is what you see at 20 feet (for Americans) or 6m (for everyone else. 20/20 is “normal” vision and is achievable for most people, although some will need glasses to get there.

It actually relates to the size of detail you can see at a specific distance, and was originally based on the anatomy of the retina. The detail of the 6/6 letters corresponds to the spacing between the cone light receptors in your macula. All 250000 of them!

The eye chart commonly uses letters, but we can also use numbers of symbols if you are a child or unfamiliar with English letters.

This is a measure of how you can see at your best, with high contrast letters in good light. It is only a partial snapshot of how well your eyes work, but it has the advantage of being simple, comparable, and consistent.