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March 1990

Equations for Snellen Equivalent

Author Affiliations

Cleveland, Ohio

Arch Ophthalmol. 1990;108(3):319-320. doi:10.1001/archopht.1990.01070050017006

To the Editor.  —I studied with interest the equation by Drs Holladay and Prager1 published in the July 1989 Archives. Their equation, Snellen Acuity = 20 × 10[(55 - X)/50], converts the number of letters read correctly from the Bailey-Lovie acuity chart2 into the denominator of the Snellen fraction for the reference distance 20 ft. The formula, which relates the base 10 to the exponential denominator 50, echoes the design of the chart, which is based on a 10-fold change in visual angle for each 50 letters (10 lines). I propose a modified equation, whereby dropping the constant 20 and taking the reciprocal yields, Snellen Acuity = 10-[(55 - X)/50], (1) where X is the number of letters read correctly from the chart. This yields Snellen acuity directly in its decimal form, which may be more useful for certain statistical analyses.Equation 1 is approximately, but not exactly, equivalent to