Twenty-twenty

Perfectly normal vision.

From the ability to score 20/20 on the test of visual acuity with the eye chart.

This familiar chart, called Snellen's chart after its inventor, is usually read while standing a distance of 20 feet from it.

Visual acuity is traditionally represented as a fraction, with the distance at which the patient is standing being the numerator, and the normal maximum legible viewing distance ('distance' on the chart) being the denominator.

So if, at 20 feet, the patient can read the letters on the row marked '20', the patient has visual acuity of 20/20