The relationship between Snellen acuity (x-axis), defined as the lowest line on which all letters were read correctly, and the ability to pass (dotted bars) or fail (filled bars) the number-plate test by reading a number-plate without error at 20 m. The relationship between Snellen acuity (x-axis), defined as the lowest line on which all letters were read correctly, and the ability to pass (dotted bars) or fail (filled bars) the number-plate test by reading a number-plate without error at 20 m. The overlap zone, or the range of visions within which passing or failing the number-plate test is uncertain, is indicated. Keziah Latham et al. Br J Ophthalmol 2015;99:545-548 ©2015 by BMJ Publishing Group Ltd.