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With imminent change to the metric system, it is important to consider suitable distances for standard measures of visual acuity to replace the 20 ft and 13, 14, or 16 in now in use. Adoption of 4 m for distance and 40 cm for near has several advantages.
Snellen fractions for a 4-m reading distance can be easily converted to those of comparable visual angle for a 20-ft distance simply by multiplying the numerator and denominator by 5. Thus, 4/16 is comparable to 4 × 5/ 16 × 5 or 20/80, and both are equal to four minutes in the visual angle notation. If a standard distance of 6 m rather than 4 m were adopted, the calculation would not be as simple because the numerator and denominator must be multiplied by 3.33.
When similar tests of acuity for capital letters are made at 4 m and at 40
Sloan LL, Enoch J. Considerations Related to 'Going Metric'The Best Metric Distances for Measuring Visual Acuity and the Correct Use of the Snellen Fraction. Arch Ophthalmol. 1978;96(9):1567. doi:10.1001/archopht.1978.03910060201001