Fig. 7. Nomogram for the calculation of visual acuity values demonstrating the relationship between the three variables: letter size, viewing distance, and visual acuity. Connecting the markers for any two of the variables with a straight line will point to the marker for the third variable. The first column identifies the letter size, in M units, printer's points, and J numbers. The Jaeger sizes are based on Jaeger's original samples.27 These differ from the ranges of J designations found on contemporary charts, as shown in Figure 14. The second column indicates the viewing distance expressed in diopters,metric units, and US units. The third column indicates the visual acuity. Notations include the Snellen fraction for 1 meter (see text), decimal notation, and US notation. The numbers in the markers indicate the visual acuity score. The graycolumns indicate the preferred metric measurements. In the range of normal and near-normal vision, the traditional visual acuity notations and distance measurement in meters are preferred. For the low-vision range, the 1-meter Snellen fraction is easier to use. For reading vision (closer than 1 meter), it is usefulto record the viewing distance in diop-ters, using the modified Snellen formula1/V = M × D. The numbers in the markers for letter size and viewing distance allow the visual acuity score to be broken down into its components: letter credit + distance credit = visual acuity score. These linear values can be averaged and are helpful for statistical manipulations.