By Masayoshi Hayashi for The Large Format Page
It's simple.
Define delta = delta on rail delta' = delta on ring D = delta'/delta N = min. fnumber required c = circle of confusion M = magnification Av = Apeture value in APEX notation Givens Goal N = delta/2/c/(1+M) Conversion from delta to delta' D = delta'/delta  delta' = D*N*2*c*(1+M) Basically that's it. Because I want to calibrate scales in 1/3 steps, using APEX notation, Av = log(N^2)/log(2)  delta' = D*N*2*c*(1+M) N = 10^(1/2*log(2))*Av  delta' = D*10^(1/2*log(2))*Av*2*c*(1+M) For nonmacro work, M is much less than 1, so making M ~ 0, giving: delta' = D*10^(1/2*log(2))*Av*2*c where Av = { 1/3 stop increments } This equation is used in making the scale. It is strictly correct at infinity focus (M = 0). For arbitrary N and nonzero M, N(M) = delta/2/c/(1+M) while N(M=0) = delta/2/c so N(M) = 1/(1+M)*N(M=0)  (1) This equation accounts for the facts explained in Good things to know section:
