Why does the subject stand 20 feet from the snellen eye chart to test distance vision?

The subject stands 20 feet from the Snellen eye chart to eliminate the effects of refraction caused by the eye's lens, as this distance is considered optimal for accurately assessing distance vision. At 20 feet, the light rays entering the eye are nearly parallel, allowing for a more reliable measurement of visual acuity. This standard distance ensures consistency in testing conditions, facilitating comparisons of visual performance across different individuals.