Western Maricopa Education Center (West-MEC) RMA-AMT Module 3 Practice Test

The distance is 20 feet because distance acuity tests use a Snellen chart calibrated for a fixed viewing distance. At 20 feet, the letters are sized so that a person with normal vision can resolve the line labeled 20/20. This standard distance keeps measurements consistent and comparable across exams and with normative data. Moving closer would make letters appear larger and make acuity seem better than it is; moving farther would make them seem worse. Many charts also note 6 meters as the metric equivalent, but the practice distance remains 20 feet in common use.