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Is there a formula that includes distance and Snellen's standard vision system?

For example, a person with 20/40 vision can read a letter of a certain size at 20 feet while a person with 20/20 vision can read that same letter at double the distance (at 40 feet). So far, I'm tempted to say that a person with perfect vision (20/20) will always see a certain sized letter as clear as a 20/40 vision person at double their distance. However, I would like an actual formula for this.

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    $\begingroup$ A small point that 20/20 vision is not “perfect vision”. A person with 20/15 vision has better vision than one with 20/20 vision. A person with 20/15 vision can see objects clearly at 20 feet that a person with 20/20 vision can only see clearly at 15 feet. $\endgroup$ – Farcher Aug 20 '18 at 9:20

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