Because of the screen size, iPhone cannot display the topmost  (Snellen Fraction 6/60, Decimal 0.1, or LogMar 1.0) optotypes at 6 meters' distance, however can certainly display the topmost  (Snellen Fraction 6/60, Decimal 0.1, or LogMar 1.0) optotypes at 4 meters' distance. Eye charts have been tested in varous iPhone models from iPhone 6 to iPhone 6.

iPhone Multiple Optotypes

iPhone Tumbling E Chart

iPhone 12

iPhone 12 Mini

iPhone 12 Pro Max Scores

iPhone SE 2020

iPhone SE 2020 Scores

iPhone 12 Pro Max

iPhone 11 Pro

iPhone 6

iPhone 8

iPhone X

iPhone XS

iPhone XS Max - About

iPhone 11 Pro Max

iPhone12Pro

iPhone 8 Plus