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Table 6 The required aliased character x-height and the necessary vertical pixel density of the displaying screen for performing the near-vision test at viewing distance D = 40cm

From: Introduction of a digital near-vision reading test for normal and low vision adults: development and validation

Actual photos of displayed text and x-height in pixels logMAR Snellen Character x-height (mm) Minimum required screen pixels/inch (ppi)
Theoretical Measured Error %
63 pixels 1.3 6/120 11.56 11.33 2.11 10.99
50 pixels 1.2 6/95 9.18 9.99 2.19 13.83
40 pixels 1.1 6/76 7.30 7.19 1.50 17.40
32 pixels 1.0 6/60 5.80 5.75 0.79 21.90
25 pixels 0.9 6/48 4.61 4.50 2.43 27.54
21 pixels 0.8 6/38 3.66 3.78 3.19 34.66
0.7 6/30 2.91 2.70 7.21 43.64
0.6 6/24 2.3 2.16 6.55 54.98
0.5 6/20 1.84 1.80 1.96 69.13
0.4 6/15 1.46 1.44 1.26 86.98
0.3 6/12 1.16 1.08 6.77 109.48
0.2 6/10 0.92 0.90 2.19 138.37
Too small character for useful display with the specific screen 0.1 6/8 0.73 N/A N/A 175.17
0.0 6/6 0.58 N/A N/A 218.97
-0.1 6/4.8 0.46 N/A N/A 273.71
-0.2 6/3.8 0.37 N/A N/A 346.19
-0.3 6/3 0.29 N/A N/A 437.93
  1. ppi pixels per inch, N/A not applicable