Millimetres, Not 'Medium': Letter Heights and Pixel Counts a Phone Must Reach for Each Level of Acuity

Phone text settings are offered as small, medium, and large, which tells a low-vision user nothing. This paper computes the actual letter heights, in millimetres and in pixels, that each level of acuity requires at ordinary holding distances, from the stated geometry that a threshold letter subtends five times the minimum angle of resolution. A user with 6/12 acuity needs a 0.87 mm letter at 30 cm - 10 pixels on a 300-per-inch display - and a comfortable reading size, at twice threshold, of 1.74 mm. A user with 6/60 acuity needs 4.36 mm at threshold and 8.73 mm for sustained reading, a size at which only a handful of words fit a small screen. The specification that follows for low-cost devices and applications is to label text settings with the acuity they serve at 30 cm, and to guarantee the 6/60 comfortable size is reachable in every reading view. This study is simulated, not measured.