Visual acuity (VA) is the measurement of how well the entire vision system can recognize what it sees. Formally, it is defined as the reciprocal of the smallest angle that the visual system can resolve.
For distance, this is typically measured with a Snellen chart and expressed as the distance you need to be from the chart to recognize the characters over the distance that someone with normal vision can recognize the characters. Lens or mirrors may be used to change the effective test distance in a smaller room.
For near, this is typically measured with a near vision test chart at 40 cm.
20/20 (VA of 1) corresponds to an angle of 1 minute of arc. The 20/20 line is 5 minutes of arc in height and width, but the smallest detail subtends 1 arcmin. 20/200 (VA of 0.1) corresponds to 10 arcmin. 20/10 (VA of 2) corresponds to 0.5 arcmin.
Reading a Snellen chart
- 20/200 would be very bad (possibly just myopic),
- 20/70 is a typical threshold of low vision (being unable to read 20/70 with best possible correction),
- 20/20 is good enough,
- 20/15 is average, and
- 20/10 is excellent.
The prescription for myopes and hyperopes is typically the weakest lens power required to achieve 20/20 or better vision measured at 20 feet or 6 meters. The prescription for a presbyope is the smallest plus power added to the distance prescription to see 20/20 at 40 cm. Emmetropic presbyopes have zero distance prescription, so their near prescription is simple spherical plus lenses.
With text that we are familiar with, the brain may clear up that text more than our vision would actually allow.
20/20 is "good enough" for most purposes, but not "perfect" in any way. Rather, 20/20 vision is slightly defective. It is possible to have 20/20 vision with 0.25 to 0.5 D of defocus, and 20/20 does not imply clear or sharp vision. As normalized correction typically imposes 0.25 D of extra myopic defocus on 20/20 correction, it can be considered to be 0.25 to 0.75 D of undercorrection.
Tscherning writes in Physiologic Optics: Dioptrics of the Eye, Functions of the Retina, Ocular Movements and Binocular Vision:
We have found also that the best eyes have a visual acuity which approaches 2, and we can be almost certain that if, with a good illumination, the acuity is only equal to 1, the eye presents defects sufficiently pronounced to be easily established.
Reasons for differing visual acuity
Non-zero refractive state, which causes blur and double vision, is only one factor of visual acuity. Various medical conditions can cause physical blockage/dispersion of light in the eye, problems detecting light in the eye, or problems with the visual processing that turns a series of electrochemical signals into a picture in our mind's eye. See an optometrist if your vision can't be corrected with refraction.
Shorthand for 20 over a single digit (such as 20/9), or considerably better than 20/20 vision (VA > 2).