To the Editor.—
The mathematics used by Dr Hammerschmidt1 in developing his final guidelines are precise and well presented. However, there are some even quicker "rules of thumb" for those of us whose calculators are low on batteries.Regarding the projected image size, the "rule of eight" may be useful: multiply the screen letter size, in inches, by eight to get the maximum viewing distance, in feet. (A 10-in [25.45-cm] letter will be readable at 80 ft [24.43 m].) Don't forget that no one should be seated beyond an angle of 30° to the screen, because most screens will reflect very little light beyond this angle.Other simple guidelines are readily available from many commercial slide suppliers, the photographic industry, and contemporary publications.2I applaud Dr Hammerschmidt for his efforts on behalf of improved slide presentations; improved slides are always worthy of applause.
Smith RP. Farsightedness. JAMA. 1985;253(9):1261. doi:10.1001/jama.1985.03350330055013