20-20 vision
noun phrase

- the ability to see perfectly, without needing to wear glasses or contact lenses:

Cambridge Dictionary

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ORIGIN

20-20 vision is the testing of vision by reading letters or identifying symbols at a distance of 20 feet (6 metres).

20-20 vision is a term used mostly in North America - in Europe, we use 6/6 notation. These figures are based on letter charts which are used in the standard sight test, such as the Snellen chart, named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862.

Many ophthalmologists and vision scientists now use an improved chart known as the LogMAR chart.

Visual acuity is dependent on optical and neural factors such as (i) the sharpness of the retinal focus within the eye, (ii) the health and functioning of the retina, and (iii) the sensitivity of the interpretative faculty of the brain.

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TALKING POINT

Ask anyone which of their 5 senses they would least like to lose and most people answer eyesight, which probably explains the abundance of expressions using "sight":

    a (damn) sight better/worse
    a (damn) sight less/more (of something)
    a sad/sorry sight
    a sight for sore eyes
    a sight to behold
    at first sight
    burst into sight
    buy (something) sight unseen
    be sick of the sight of (someone or something)
    catch sight of (someone or something)
    come into sight
    drop out of sight
    get (someone or something) out of (one's) sight
    have (someone or something) in (one's) sights
    hindsight (Nachsicht, Sicht im Nachhinein) is always 20/20
    in one's sights
    keep out of sight
    keep sight of (someone or something)
    know (someone or something) by sight
    look a sight
    lose sight of (someone or something)
    love at first sight
    lower/raise one's sights
    no end in sight (to something)
    not a pretty sight
    out of sight
    out of sight, out of mind
    second sight
    see the sights
    set (one's) sights low/high
    set (one's) sights on (someone or something)