On Mon, 24 Dec 2018 22:33:39 +0100, Paul Schlyter
wrote:

On Mon, 24 Dec 2018 09:26:32 -0500, Davoud wrote:
20/20, or metric 6/6 = 1. It's explained at
https://en.wikipedia.org/wiki/Visual_acuity#Measurement.

Ok, so it measures the visual acuity at a distance of 20 feet. But
wouldn't a measure of (practically) infinite distance be more
interesting to skywatchers? No stars are only 20 feet away.

The distance is largely irrelevant. It's measuring resolution... the
ability to detect close high contrast features as separate from one
another. The distance of 20 feet is simply a standard so that the test
chart is always the same, and it's a distance that is practical in a
typical testing situation. Optically, 20 feet is pretty much the same
as infinity for the human eye in terms of accommodation.

Modern refraction techniques project patterns on the back of the eye
and directly assess accommodation, focus, and astigmatism. But this is
usually translated to the 20:X notation for simplicity.