More in this test report.

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 6

iPhone XS

iPhone X

iPhone XS

iPhone XS Max - About