Reading Additions Derived, Not Guessed: An Age-and-Distance Table from Hofstetter's Amplitude with Half Held in Reserve

Ready-made reading spectacles are sold by age-banded guesses, and the guesses are rarely shown. This paper derives the banding from a published empirical relation - Hofstetter's average amplitude of accommodation, 18.5 minus 0.30 times age, in dioptres - combined with the stated working rule that half the amplitude is held in reserve, and computes the reading addition for ages 40 to 65 at three working distances. At 40 cm the first addition appears at age 50 in this tabulation, reaches 0.75 D at age 50 and 2.25 D at age 60; a 33 cm working distance demands about half a dioptre more at every age, and 50 cm about half a dioptre less. The resulting selection rule for age-banded stock is printed in full, with its error: wearers whose amplitude sits at Hofstetter's minimum rather than the average need up to three-quarters of a dioptre more than the band gives, so banded stock should always include one stronger pair in each size run. This study is simulated, not measured.