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Five Arcminutes

The Snellen eye chart is based on 5 arcminutes angular resolution (1/12 degree) as the reference level for normal vision, i.e. using the small angle tan approximation, this is being able to discriminate an 8.8 mm object at 6 m. 6/6 (or 20/20, if you prefer feet) is “normal”. If you can discriminate only 17.6 mm then your vision is 6/12 (i.e the object would have to be placed at 12 m for reference angular resolution).

 

The following table approximates acuity if object is at 1 m.

vision2

 

Legal blindness: despite maximal refractive correction the acuity is <= 6/60 (20/200).

In the following graphic, a the distance a normal person could just discriminate the smaller E, a legally blind person could not discriminate the bigger E.

vision

 

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