Visual acuity
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Visual Acuity is the measurement of how well the vision system as a whole is recognizing what it sees. 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, so
- 10/400 would be very bad,
- 20/20 is (by definition) normal, and
- 40/20 is excellent.
Refraction is only one part 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.